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THE ANORTHOSYTES OF THE Minnesota Coast of Lake Snperior. II. THE LACCOLITIC SILLS OF THE North- West Coast of Lake Superior. BY ANDREW i\ LAWSON, ASSOCIATE PROB'ESSOR OP GEOLOGY AND MINERALOGY, IN THE UNIVERSITY OF CALIFORNIA. WITH A PREFATORY NOTE ON THE NOBIAN OF THE NORTHAVEST, BY N. H. WINCHELL. MINNEAPOLIS: HAKRISON & SMITH, STATB PKtNTERS. 1893. :/ GEOLfxaOAI. AND NATt'KAL HISTORY SURVEY OK MINNESOTA. N. H. WINCHELL, State Geologist. BULLETIN \(K il. T. THE ANOP.THOSYTES OF THE Miuiiesotci Coast of Lake Superior. IT. THE LACCOLITIC SILLS OF THE North-AYest Coast of Lake Superior. BY ANDREW V. L.VWSOX. ASSOCIATE PROFESSOR OF GEOLOGY AND MINERALOGY, IN THE UNIVERSITY OF CALIFORNIA, WITH A PREFATORY NOTE ON THE NORTAX OF THE XORTHWEST, BY N. H. WINCHELL. MINNEAPOLIS: HARRISON & SMITH, STATE PRINTERS. I89;t. CONTEN1\S. THE NOKIAN OF THE NORTIIM EST. PREFATORY NOTE. Introduction. IN'tni^'raphiciil Chiiraotnrs: Irvine's Description. Supplt'racntary Description. Tile Constituent Feldspar. Optical Measurements. Contlrinatory Tests. Rock Composed of Labradorite. Rock Composed of Anorthite. Chemical Analyses. The Name Anorthosyte. Accessory Constituents of the Anortho.syto. Interp(»sition8. Distribution and Mode of Occurrence of the Anorthosyte- Two Modes of Occurrenc'e. Occurrence near Encampment Island. Vicinity of Split-rock Point. Irvin(;"s Views on the Occurrences at Split-rock. Winchell's \'iews. Anorthosyte confounded with Keweenlan Eruiitives Occurrencesat IJeaverBay. .Shore below 1 leaver Hay. liaptisru River On tlie s|(,i)e of Saw-Teeth. Carlton l\!ak. GeoloK'ical relations of the Anorthosyte. Pre-Keweenian a^'e I)ome(l and humiuocky character of the pre-Keweenian surface Interval o! erosion. Al)sence of tlie Animikie. Shallowness of the Kewt!i;nian. Correlation and name of the formation. Introduction: General note. Earlier descriptions. Dissent from former views. Views here advanced. Petrographical character of the trap sheets. Constancy of character. PetroKrapliical ditl'erentiation. Some broad features of the trap sheets: Their simplicity. Absence of pyroclastic rocks. Absence of How structure. The enclosing rocks. • Intersection of strata by the sheets. ;• Passage of sheets to tlie horizon of the Keweenian Lower contact of the Trap Sheets. ' * Upper contact. Alteration of enclosing rocks. Summary. Geological consequences. 1 \ THE NORIAN OP THE NORTHWEST. PKEPATOUV NOTE BY N. H WINCHKf.T.. Among the problems which were named in instructions given to Dr. Lawson when he entered upon the season's work for the Minnesota Geological and Natural History Survey along the northern coast of lake Superior, one was the following — To determine if possible the date and stratigrapliic relations of the gabbro invasion. How well he has answered that question his report on the "Anorthosytes of the Minnesota coast of lake Superior" bears ample testimony. In order, however, that the imjiortant bearing which his results have on the geology of the northeastern part of the state, may be made more ai)parent, it is deemed best to preface this publication with a short exposi- tion of the history of the investigation, and a few i)aragraphs on the extent of the gabbro rocks and on the significance of the term gabbro. The reports of the late survey of Wisconsin, particularly those portions contributed by professors Pumpelly and Irving, treat largely of a terrane which they placed at the bottom of the then called "Keweenawan", and which they designated by the general term gabbro. This name had already been given by Prof. Streng to the "Rice Point rock" which is seen abund- antly at Duluth,* and by Dr. J. H. Kloos in 1871, who had pub- lished his preliminary field-studies. + Again in 1877, Dr. Kloos gives further details of the distribution and microscopical char- acter of this rock.]; He calls attention to "two totally unlike crystalline rocks" at Duluth, one of which is the "gabbro or hyi)ersthene rock", which he says had since the examination by Prof. Streng, actually proven to be gabbro, and which has an "enormous preponderance of labrador plagioclase", and great paucity of other elements which are "with difficulty de- tected between the feldspar crystals, and can only be dis- tinguished with sufficient clearness. in thin sections". The • Neues Jiilirbueli, IH77. See a, trausliitioti in tlie Ek'vont\i Minnesota report 1882. t Zeitschrlft der Deiitselien Ueoiosist'heii Genellscliaft. p. 417; triiiisliitlon in tlie Teiitli Minnesota report. Compare pajre 1!>4. where tlie Duliitli rock is said to liave •'a reseml)lance to gabbro. or liyperstlieny te". % Zeitsolirift der Uesellscliatt fllrErdUurKle zu Berlin. Bd. XII, 1877. Translation in tli.e Nineteenth annual report. Minnesota survey, pp. 81—121. IV HULI.IiTIN NO. VI 11. other cry'^tfillinc rock dnst^riltcd by l\loos al Diiliitli is tho •'porphyry-lilco mt'lapliyr". whicli appears ii(;ar tlio lake slioro. Ho calls attention to tlie contrast wliieli it i)rosents to th«? ^'abhro, and to its similarity with I lie rnck at St. (Jroix Falls. Tliis rock is not only i)orphyritic l)ut also amyj^'^daloidal. The exa<'t field relati()n> l)ei\veen these two he (!ould mA niakc^ out. altlHiu^''h it has since been asc(!i"taiiied that the nielaphyr lies above the tr()<,'i"i|)lii(ally with thosi' of llic K«'W sub- ordinate importance of the foliated condition of augite. ••by which gabbro is ordinarily separated from diabase, of which it would seem to be merely a phase". Yet Irving retained the name, both in his descriptive text and in his maps, "not only because most of our rock is very close to the typical Euroi)ean gabbros, but more especially i)ecause it is so sharply contrasted with the typical Keweenawan diabase that a separate name seems necessary." He found no indication of bedding like that seen in the diabase. He found tln> gabbro cut by intrusive granite, much of it being tine-grained and pinkish mixtures of orthoclase and quartz. * There Is a u'ciioral rlcscriptloii (if tills •altcniiitldn" cm pp. I'iT— 7S. Vol. III. flroloify of Wlsc-onsin, wliete it appeiirs Dial I tiere was ohsi-rveil iiotlilii;; more tliuii an inter- rupted series of exposures of iiabbri). with diabase iiilerveiiliii:. < Irviui;, Geology ot Wlseonslii, Vol. Ill, p. ITI. VI BULLETIN NO. VJH. Irving put lln' j^abbro in Um? Kowoonawan for tlio following reasons. ((Jool. Wis. Vol. III. i)p. 171 171'). 1st. - Th«' clo.so similarity it pni.sonts in min»»ral coinpo.sition to tho truo K«'\v(M»na\van diuba.s'^. -nd. The cvidont intorstratitioation with the latter noar tho .nmction of tlio two. ;irxtonsion of tin' j,'ubl)ro bolt in the vicinity of Montreal river. 1th. The niassivenoss and apparent eruptive nature of the ^abbro. 'ith. Tlie occurrence of «,'abbros in tho typical Keweonawan lejjfion of K(;we(»naw point, and with tho typical Ivewoenawan diubasc! in Doii^'las county, Wisconsin. (ith. — The appai'ent non-conformity of tho j^abbro and the Huronian schists, as indicated by the vvay in which the junc- tion lint! l)etween the two cuts diagonally across the strike of the Huronian Ijods. In tho description of these «?al)bros both labradorite and an- orthito are marly unchang<'d. One tield exposuri? is described as "light-gray, often nearly w\iite gabbro, a peculiar phase not noted else- where. Under the micro.scope the largo i)lagioclasos make up nearly tho whole section." Indeed both lie and IMuupelly refer in several instances to the exceeding coarseness of the plagi- oclase ci'ystals. and the great })reponderanco of the feldspar over all tlu; other constituents of the rock. The third annual report of the director of tho United States geological survey contains a somewhat later ( 1HM3) exposition of th(! Keweonawan by Prof. Irving, embracing some now field observations and some new lilhological studies. Here ho still retains tho gabbro series, whether in Wisconsin or in Minne- sota, in the Keweenawan; announces a new variety of tho gab- bro series from Minnesota, viz.: "anorthite rock." which he says is "mei'ely a coarse gabbro. in which all the ingredients but the feldspar are wanting." (p. 07) and gives some of the de- tails of (di iinc()>if(ir)iiitij oj the ffabltro sefies Ijelow the diabases of the rest of the formation, in the Bad river region. This jJassage is so imjwrtant that it is worth quoting in full: "On Bad river, eighteen miles southwest of the Montreal, the lower division [/. e. the bedded diabases] has a surface width, from the Huronian slates below to the sandstones of the upper Ik; THE NORIAN OF THE NOKTHWKKT. VII division, of only 17,000 foot. Sinco tlio (lip horo is porpondic- ular, or nearly so, the thicknoHS is not much Iosh than this. It is shown, in the ori^final nionioir [Monograph V, U. S. Gool. Surv«;y|. that this oxtraordiiiary thinniri«r is coriiioctod with the proscnco ht'low of a ^roat holt of tiio coar.so <;al)l)ro dos(',i"il)od in a procodin^ pai'a^rraph of this cliaiitor. Tliis ooarso <.,'al)l)ro — whethorwith or without int(ul)od(U'!d tine-Ki"vinod bods is not now Ifnown— usurps most of tho thicknoss. ioaving only some 5,000 f(»ot for tho usual thin boddod flows of tho lowor division. Tho oxplaiiation may Ijo that, oarly in tl\o history of tho sori»!s theri! was ]>ourod out horo an immonst; thiclcnoss of a rock which soliditiod into the coarse j^'aijhro. whilo later in its f,'rowth tho vonts were removed from here to either side. The coarse gabbro mass must have stood up to a groat hight, and tho later flows toi-minatod against it on oithor sidc^ until they had accumulated sutticiontly to overflow its upper surfacu," (pp. 130-7). On page HM, also. Prof. Irving si)oaks of those coarse gray gabbros. stating that they "present the appear ance of a certain sort of unconformity with the overlying bods.'" It a])pears from this that profossor li'ving was not willing to state un([ualitiodly that tlie galibro and tla; ilial)ase were inter- bedded in tho Bad river region. It also app(>ars that in the same region the same relation of nonconformity obtains as in Minnesota, although its significance was not fully entertained by Irving. Thus Irving saw some? of tho evidences adduced by Dr. Lawson lor considering the gabbro n^cks ol" Wisconsin and Minnesota as constituting a formation intermediate be- tween the diabase phase of the Kewoonawan and the formation which he styled Huronian, which latter is a far older terrano. b(,'ing, undouljtodly. one of tho series which we now know as Ontarian. and probably the lower. It was owing to Prof. Irv- ing's confusit)n of rocks of several distinct horizons under the general term Huronian that he fell into the error of putting the gabbro into the Keweenawan, an error that has vitiated the work of some of his as.sociatos. and that of some of his follow- ers, and wiiich projected itself into the geology of Minnesota. Whereas he saw the gabbros overlying and traversing the strike of the ••Huronian" (Ontarian) in the region west from Bad river Wisconsin, he inferred, they must be later in date. Again, when he found the '•Huronian"(Animike) associated with rock "identical with the Duluth gabbro." on Pigeon point, in a manner not readily determined, he applied the conclusions of mm iv VIII BULLETIN NO. VIII. the former obsor\'ation, and was moved to place the gabbro above the Animikio, at the base of the Kevveenawan, although there is at that point no evidence whatever looking in that di- rection. Had he understood how much lower in the strati- graphic scale the Bad river "Huronian" is than the "Huronian" with which he compared it in other parts of Wisconsin and in Minnesota, he would probal)l3' have given due heed to both of the \inconformities which he briefly notes, and would have re- tained the gabbro in that stratigraphic position, where for some reasons he had felt inclined to put it. It is. perhaps, due largely to the researches of professor Irving that the appli'3ation of the term gabbro in the Northwest was extended beyond the rocks that possess the petrographic characters at first included under it by Kloos and Streng, and by Pumpelly. Irving found all conditions of change in the augite toward diallage, and all conditions from diallage to chlorite.* The chief distinction between the gabbro and the diabases of the "lower division" of his Keweenawan consisted in outward structural features, and in visible petro- graphic characters. The gabbro is anbedded; the diabases always evidently bedded. The gabbro is generally light-colored and coarse-grained, never amygdaloidal, the diaba.ses are dark, fine-grained and frequently amygdaloidal. Although he main- tained that the gabbro was extravasated as a surface flow, and "cannot have been intrusive", yet he noted many of the evi dences that disagree with that view. In short, structurally and petrographically the gabbro, in all respects.was thought to grade into the Keweenawan. This view was announced in his Wi-sconsin reports, and his later work for the United States Geological Survey does not vary in any of its main results, from the conclusions of the Wisconsin reports. When we look into the reports of the Minnesota survey we find the term gabbro applied to the Duluth rock first in 187!) (published in 1880), at page 23. The present Avriter began a preliminary microscopical examination of a series of rocks col- lected by the survey, mainly along the shore of lake Superior, and in August, 1881, he read a paper before the American Association for the Advancement of Science, t in which this rock is referred to, but is included in the Cupriferous series. This was based on field examinations conducted in 1878 and 1879. * Third iiiiniial report of tlic director of tlic I'nlted State OooloKlcal Survey, p. 10). + Clneiniiiitl meeting, p. IfiO. Tenth Annual Report. Minnesota Survey, p, t37. THE NORIAN OF THE NOHTHWEST. IX As noted by Dr. Lawson fp. 14) the gabbro and feldspar rock at Beaver bay and about, Split-rock poi it were referred, con trary to the opinion of Norwood, to a date older than th(» traps forming the shore. "Large masses of feldspar rock, (embraced in the trap, as boulders are embraced in hardpan clay, have been carried from Carlton's peak, or from a range of hills north and west of it. toward the east and southeast. These embraced pieces become smaller in going from their place of origin, in the same manner as fragment 5 of rock acted on by the drift forces." ( Seventh Minnesotfu rei)ort, pp. lL^13). It was stated that the "feldspar rock" and the gabbi'o at Duluth are of one and the same formation, this statement being based on the existence, in minute quantities, not only of all the gab- bro ingredients in even the feldspar masses, but the existence in the pudding-stones where the feldspar masses prevail of some rounded gabbro masses showing various degrees of trans- ition to pure feld.spar. the augite and magnetite being evident to the unaided eye, the former frequently chloritic. The writer had also examined the gabbro range at points remote from the coast line, and had expressed a similar conclusion. The following is from the tenth annual report, p. 80: "The mineral composition, however, is constant, and allies it to the Igneous rocks of the 'gabbro range" which graduate into the trap rocks of the Cupriferous. It seems, however, that there was a vast outflow of igneous rock in the midst of the era of the quartzyte and slate group producing this area of the gabbro range, and separating the lower portion from the era of the porphyries and the red shale and the red sandstone which are characteristic of the Cu])i-iferous formation proper. There are some reasons for believing that this great igneous outflow entered the sedimentaries as laccolites in many places and thus tilted and modified the overlying beds, instead of being produced prior to their deposition." Again on page 99. tenth report: "On the south side of Little Saganaga lake the rock weath- ers white. It rises in higher bluffs, resembling the Rice point gabbro, and even approaching the whiteness and nearly the purity of the so-called 'feldspar rock'. It is mainly of feldspar, but also contains magnetite and a little pyroxene. The hills and ridges in general show a coarse bedding which dips south. Ridges 10 to 50 f(»et high. The sample collected is weathered. * * * * A pinkish-red vein of syenite, 20 inches wide, cuts this gabbro. It is comparable to the red syenite X BULLETIN NO, VIII. associated with the ?lst, March, 1800. vol. v. p. 17-t. , THE NORIAN OF THE NORTHWEST. XI II This quartzyte sometimes is impui'e an 1 limonitio. and seems to be the cliiof iron horizon of the Mosabi range. This near conjunction (which is sometimes apparen ly an exact contact) of the «^abbro with the gneiss, and the absence of tj^e Animike proper between them, has been supposed to be due to a local overlap of the gabbro beyond the strike of the Animike, cover- ing it from sight, the idea being that the gabbro flowed back northward over older formations, and came on to the gneiss. 2, Although there has not yet been any careful microscopical examination into the ditferences between the typical gabbro (for instance that seen at Rice's point, near Duluth) and the eruptive rocks that overlie the Animike black slates at Gun- flint lake and eastward to Pigeon point, it has been noticed that there are microscopic distinctions which ought to be explained in case of a supposed parallelism of one rock with the other. The supposition has been that they are stratigraphically and chronologically the same, and that the differences were only local and imimportant. It was this assumed parallelism and the evidently later age of the eastern outcro|)s (the "crowning overflow" of the Animike) which has led to the placing of the gabbro later than the Animike. There is absolutely no other evidence. If these two eruptive rocks are not contemporai'y, there is not only no reason against, but considerable evidence in favor of placing the typical gabbro (such as at Rice's point, and at Little Saganaga lake) below the body of the black slates. 3. Boulders of characteristic gabbro and of red syenite, and of the quartz porphyry, occur abundantly in the later "traps" of the Cujiriferous. The quartz-porphyry pebbles are so abundant as to constitute the well known thick beds of coarse conglomerate; and quartz porphyry layers or lenticular shceta are interbedded between the trap sheets. This quartz-porphyry in some cases appears to have originated in its present condi tion of interleaved sheets during the time of the Cupriferous. This is observable at the mouth of Baptism river, and at the Great Palisades. At these points, however, owing to the prox- imity of bosses of gabbro rising above the rest of the country about, it is certain that those portions of the Cupriferous, which contain the original quartz-porphyry beds, are near the bottom of the formation. This is further shown by the exist- ance, in the same region (at and near Beaver Bay), of large boulder-masses of gabbro in the trap flows, evidently derived from the neighboring gabbro hills. Prom this point, north- ward to the gneiss of the Giant's range, nothing is visible, in XIV BULLETIN NO. VIII. the form of rock in situ, except gabbro, or some * "muscovado" - like rock described at some outcrops somewhat further west* by H. V. Winchell. The region is not fully explored, but it appears frc^m all that is known, that there is nothing to be found of the typical, thin black slateS of the Animike. It is as reasonable to infer that they followed after the gabbro flood, as that they preceded it. In case they followed after it, their typical characters were destroyed in this region by the fre- quent outbursts of igneous eruption, and they blended with the tuffs and shales and basic sheets that constitute, on the north shore of lake Superior, the lower portion of the Cupriferous formation. In case they preceded they must exist buried be- low the gabbro, as hitherto supposed. # * * * 5. On the supposition that the Animike black slates are in- volved in the Keweeuawan, and, while overlying the gabbro, lose their typical characters at points further southwest, the interbedding of the Animike with beds of trap-rock, which is a common feature about Gunttint lake and on the shores of Loon lake, is easily explained, and indeed falls into place as one of the to-be expected facts of such a period of recurring erup- tions. It also obviates the neci ssity of a supposed change in the character of the eruptive rock, i. e., that the gabbro of Rice's point and Little Saganaga lake becomes, on Pigeon river, the dark or greenish trap-rock and the dioryte which in- ter-bed and characteristically overlie the Animike, forming the well-known 'crowning overflow' of that region." Again, in "The Iron Ores of Minnesota,'" there are some con- siderations touching the nature and date of the gabbro dis- turbance, f "The rock itself is gabbro, a basic eruptive, of gray color and generally of coarse crystalline texture. Its minerals are labradorite, augite, magnetite, biotite, olivine. The relative amounts of these minerals undergo great variation. While perhaps in no case will any of them be found entirely wanting over large areas, they are severally sometimes so scarce, while at the same places some of the others prevail, that the rock takes on very contrary aspects. When the labradorite pre- vails, as about Little Saganaga lake, and Bellisima lake, and in Carlton's peak, and in the feldspar masses that are embraced in the dark trap at Beaver bay, the rock when fresh is glassy, •Seventeenth report, p. 00,91. Samples 387 (II) . +The Iron Ores of Minnesota. Bulletin VI, Minnesota Geological and Natural His- tory Survey, 1891, p. 12a. THE NORIAN OF THE NORTHWEST. XV J gray, and firm, but on weathering it becones almost white. When the magnetite prevails, as in the svburbs of Duluth, about the southern environs of Birch lake, at Iron (Mayhew) lake, and many other places, the rock is black and firm, and simply becomes speckled with lighter spots on weathering, the spots indicating the existence of crumbling crystals of labra- dorite. When the olivine or augite or both prevail, which is apt to be accompanied by the appearance of crystalline masses of hornblende, and in cases of weathering near the water, the rock has a green, or dark green color, the green tint being in- creased by the conversion through weathering of some of these into serpentine, chlorite or delessite. * * * It was supposed by the geologists of the late Wisconsin geological survey that the gabbro eruption in the main, took place after the completion of the Animikie strata, and that it formed the base of the Keweenawan, fading off upwardly by a succession of traps and sandstones and with interbeddings of conglomer- ates and volcanic tuffs into the most characteristic features of the Keweenawan. This view has also been held by all the Minnesota reports except the eighteenth. But it has been found that the great gabbro flood of northeastern Minnesota was outpoured at an earlier date. In the sixteenth annual re- port* will be found evidence that it began during the deposi- tion of the Pewabic quartzyte * * * ^ and must follow it to the lower portion of the Animikie, and hence to near the commencement of the Taconic." On the geological map which accompanies * 'The Iron Ores of Minnesota," the gabbro belt is roughly laid off and represented as a distinct terrane separate from the diabases and conglome- rates of the Cupriferous proper, under the name Norlan. By an omission in the "transfer" by the lithographer the Carlton's peak area was not represented as Norian. although the Beaver Bay area of feldspar rock was included. This separated part was designed to include also all the red syenites, diorytes and felsytes with which the gabbro rock is closely associated. In thus removing large masses of crystalline rock, usually con- sidered to be portions of the Archa3an from the Keweenawan, it was not intended to convey the idea that they should be placed in the Archiean. It had been a growing impression, and had reached almost a conviction, resulting from an attempt to comjjare and adjust the results of study on these rocks in Min- nesota with the published results of the Wisconsin survey, that ages 85, 88; 17th report, pp. 5a. 53; 18th report, pp. 43-17. XVI MULLKTIN NO. VIII. thoro was a ?radations to tyjjically constituttfd <,'al)bro. Tliere are also, am(»n«^st th(>se detached masses, some tiiat are darker than the ordinary gal)l)ro, containing very lari,'e ))roportions of augite and magnetite, with some olivine J It thus appears tliat at the Iocks, where the later diabase sheets received the distinctiv*^ anorthite rock, there were variations to typical /^ab- bro, and even to the very dark and heavy <;abbro. S\ich vari- ations have been noted repeatedly at p<»inls where the f,'abl>r() belt appears in Wisconsin and in Minnesota. Aj?ain the red granite i)ieces, which appear as isolated bould(n's in the diabase tlows at Beaver bay, indicate the same relation, for red ^'ranite. the augito syenyte of Prof. Irving, is a fre(|uent att»'ndant of the gabbro range in Minnesota, as well as in Wisconsin. The nicer distinctions between the feldspars, i. e. whether hibradorite or anorthite, do not seem to play any im])ortant part in the solution of the question at issue. There is good authority for the statement that both those feldspars are found both in the anorthite rock and in the gabbro. al- though it now appears that labradorite predominates in both. This itself is a bond of petrographic alliance between them and is perhaps as strong as any. Negatively considered, if the anorthite rock be not the same formation as the gabbro, room must bo made for both of them in the geologic composition of the northern part of the state, and it will require a greater reconsideration and a great- er reconstruction to accommodate two such formations than to accommodate one and implies greater error in all previous work. The consequences of this important result are far-reaching. It makes it necessary to reconsider some of the descriptions that have been published of the relations of this rock to other formations, in Wisconsin, and also to make some comparisons with similar geology in regions further east. The Bohemian mountains, of Keweenaw point, were described by Poster & Whitney]: as constituting the axis of an anticlinal ♦See pp. ni, ns and IIU. Tenth Annual Report. +0ne of the samples collected l)y the writer, numbered 818, shows this character. Anotlier of the same number is nearly pure feldspar, but Us grain Is not so fresh and coarse as mucli of titat seen at Beaver bay. iUeport on the Geology and Topottrapliy of a portion of the lal{0 Superior land dis- trict. I'art 1. Copper lands, p. 64, 1850, XX nuLi.irriN NO. viii. I Htructurn, which thoy gave to tlio poiut. The rock of this r:inj;<' wiis said to he difTcrcnl from that in tlio iiorthiM'u trup raii^fcs, l)otli in litliolo^ical cliaractci" and in tho luoch' of its occurrence. Tiic northern ran^i! was nuide up of numerous trap V^t^ds, with amyj^daloidal and ^'ranuUir varieties intorstru- titled, ami with various dotrital rodcs. especially con^iomei-ates. The Bohemian, or soutiiern ran^je. consisted of a vast crystal- line mass, forniinf; an anticlinal axis, "tlank'cd on the north by tiie l)edded trap and con^'hjmorale, and on the south by con- glomerate and sandstone." The contour of the bedded trap was observed to be very dilTei'ent I'roni that of the unVjedded. The hills composed of the former rise by a succession of stair- like ascents; those of the latter are either dome-shaped or rounded. The i-ock itself sometimes (-ontains numerous crys- tals of maj^netic ii'ou ore which occasionally torms a lar^e por- tion of the rock. The diajjfi-ammatic section given by Foster & Whitney (p, 00) shows Bohemian mount, "comi)osed of labra- dor and chlorite," with its irr(>f;ular patches of mai^netlte, forming a central range in the Uivm of an anticline from ic/iich lilt' (itlicr rocks 'm intcnn/nniKih/t on'riic, dip in o])posite directions. Numerous geologists, including Jackson, Foster and Whitney, Whittlesciy, T. S. Hunt, Gaujot, described the structure as anti- clinal. ltapi)ears that this structure is very similar to that found to obtain in Wisconsin and Minnesota, whei'i'ver these terranes exhibit their mutual relations, and it provokes tlie in(|uiry whether it may not be well to reexamine tlie geology of Ke- weenaw point with this (piestion in view. Prof. Irving included these rocks all in on(». series, substantially conformable amongst themselves, under the term Keweonawan, but it appears more than probable that there is a profound break between the rocks of the Bohemian range and the well Icnown traps of the north- ern range, and that too mucli was embraced under the name Keweenawan. THE NORIAN. The foregoing descriptions lead irresistibly to a search for a suitable name for the lower series. Precedent and scientific nomenclature seem to require the use of an old name if it be found that one has already been given to this group of rocks. Fortunately the dominant characters of this group are very marked, and they are easily found when they have been ascribed to any other locality. As we approach nearer a grand classi- fication of the crystaline terranes, such as can be said to be ap- plicable over large if not continental areas, it will become THE NUlilAN «)K T IK NUUTHWKST. XXI necessary to abandon sonic of the local natnos which have benn nsod (Inrin^r the coui'sc of invcstijfation. and to clioosc from the various synonyms those which will l)c wai-ranlcd i)y the rules of ^('olo^ical nomcnclaturci. Mr. I^awson has proi)os«jd a n«'W name (Carltonian) and that will bo convenient for those who do not yet feel ready to accept the ^'lander parallelisms, and who do not wish to confound their studies by the use of possildy aml)i^'\ious terms. Hut to the wi'itei- thi> ti«'ld study and the reseai'ch have been pursued far enou«^h to warrant the refer enco of these rocks to a nico^ni/ed epochal position, with a welllfnown desififnation. At tirst calhnl upper L.iurentian. by Logan, because they \\(} niiconformablt> on the i-oal Fjaurentian rocks, north from Montreal; sul)S('(|uently rjubradoi-iaii. by T. Stei-ry Hunt, because of the prevalence of that mineral ( lal)ru- dorite) which received its name from Labrador; they were lastly designated, by the same r Laurentian. Were it not for this th(i term ••upi)er Laurentian." having precedence in date, might be perpetuated. Dr. Hunt was the author of both the other terms, and his later choice was Norian.* His succinct description of them is in the following terms: "The typical norytes consist cliielly of atriclinic feldspar, varying in comi)osition from anoi"tnite to andesine. but gener- ally near labradorite in comi^osition. The color of tliese rocks is ordinarily some shade of blue,— from bluish-black or violet to bluish gray, smoke gray or lavender, more i-arely ])assing into liesh-red. and occasionally greenish-blue, greenish, or bluish-white. The weathered surfaces are opaque white. These norytes are sometimes nearly pure feldspar, but often include small portions of hypersthene. pyroxene, or horn- blende, --the former two being sometimes associated in the same specimen and in contact with each other. A black mitja (biotite), red garnet, epidote, chrysolite, and menacannito (titanic iron) are frequently present in the.se rocks; rpiartz. •Chemical and Geoloj?lciil essays, Tliird edilioii, 1891, p. t,*?**. The term was also ujed by Hunt In the second edition p. 278. 1878. XXII BULLETIN NO. VIII. however, is rarely seen, and then only in small quantities. Through an admixture of the first named minerals these norytes pass into hyperyte, diabase and dioryte. The norytes vary in texture, being sometimes coarsely granitoid, and at other times tine grained and nearly impalpable. The coarser varieties often present large cloavable masses showing the striic char- acteristic of the polysynthetic macles of the triclinic feldspars, and sometimes exhibit a fine play of colors, as in the well- known specimens from Labrador. A gneissic structure is well marlcod in many of the less coarse grained varieties of noryte, and the lines of bedding are shown by the arrrangement of the various foreign minerals. Although norytes predominate in the Norian series, they are found in the area of these I'ocks which is seen te the north of Montreal to be interstratitied with beds of micaceous orthoclase gneiss, quartzyte and crystalline limestone."' Toward the south further these rocks prevail in the south- ern Adirondacks, appearing on lake Champlain. They contain in Essex county the titanic iron ore which has been mined, for many years. They are here associated, in some manner as yet unascertained, with a large series of ([uartzose gneisses, crys- talline limestone and hematite ores. In the northern slopes of the Adirondacks these latter rocks are very largely developed, dipping conspicuously toward the north, and northwest. A late examination by the writer convinced him that these gneisses do not belong in the true Laurentian. although usually so regarded.* In some of their outcrops they seem to pass litliologically into the quartzyte so largely wrought at Potsdam, although no structural evidence was found to support sucli a hypothesis. In many places, however, in eastern New York the norytes have been described as interbedded with such rocks, and in the Court landt series, on the Hudson river they embrace nonconformable masses and blocks both of quartzyte and of limestone known to be of primordial age by the dis- covery of characteristic trilobites. The series extends into N(.N\ Jersey, carrying valuable iron ores, long mined, but there the mined ores are uon-titanic magnetites. In the Courtlandt series, according to the descriptions of Prof. Geo H. Williams f all transitions occur between the degree of presence or absence of the hornblendic, the augitic, the biotitic and the chrysolitlc varieties. The olivine bearing portions are "destitute of any "iii'e, however. , I A.MES llAi.i,. Am. .lourii. Sci., (;i). Xil, 2W, who e.xcliulos tliotii from tlio Laiiroiitiiui. tAnierlciin .loiiniiil of (•'ch'iR't! (III). XXX. .Imii. li^J-e, p. 27. THE NORIAN OF THE NORTHWEST. XXIII considerable quantity of feldspar, and belong therefore to the family of peridotytes". They pass, however, by an increase in the amount of feldspar, into olivine norytes, olivine gabbros, and olivine diorytes. "The constant occurrence of such trans- itional forms, and the vi'ant of any regularity in the distribu- tion of the pure tpyes. make it impossible to regard these rocks as anything but local moditications, or special facies of one and the same mass. However great their mineralogical variety may be they together form but a single geological unit". The age of this invasion of the fragmentals at Courtlandt by basic irruptives is certainly i)aleozoic. on the authority of Prof. J. D. Dana;* and Prof. J. F. Kemp has stated that they contain (at Rosetown near Courtlandt) masses of Cambrian limestone. Dana, indeed, at first considered them metamorphosed sedi- ments of Lower Silurian age. as there are various crystalline effects ijroduced by them on the concerned elastics, resulting in schists and diorytes and soda-granites. The same rocks have been found by C. H. Hitchcock in the White mountains of New Hampshire, and there they are said to lie non-conformably upon an older metamorphosed series, which again is non-conformable upon the true Laurentian. A formation thus characterized and so easily identified by its lithology as well as its stratigraphic relations should not long- er be without a recognized name; and it seems appropriate that it should bear the name Norian. whether it be found in the east or in the northwest. In Minnesota, and in the north west generally, wherever these rocks have been found they are associated with various evidences of upheaval, metamorphism and eruption. Therefore, they afford not only one, but numerous interesting features, which are as yet but faintly understood, giving rise to several problems, the true solution of which can be but partially foreshadowed, or are en- tirely beyond answer with our present Ivnowledge. THE LOGAN SILLS. Professor Lawson's second paper, the. laccolitic sills of the Anl- inikie rocks of lake Superior, has a close relation with some of the problems alluded to abo^'e, and his investigation throws some light on them. It had been, apparently, the source of some perplexity with Prof. Irving, that the gabbro, wherever he saw it in Wisconsin — and the same is true of it in Minnesota — is non-bedded, non- ♦Amerlcan .lourtiiU of Science, (HI), XX, 191. 1880. XXIV BULLETIN NO. VIII. amygdaloidal, non-basaltic, never presenting the peculiar stair- like ascents up the hills which it forms, noted by Poster and Whitney as characteristic of the traps of Keweenaw point. Ir order to account for this, and for some other peculiarities of its distribution and manner of outcrop, he imagined that perhaps the gabbro bosses that now appear at the surface are simply the congealed and now uncovered reservoirs of basic rock ma- terial which furnished the diabasic overflows of the Cuprifer- ous. This suggestion he repeats: "The great coarseness of grain, the perfection uf the crystal- lization, the abrupt terminations of the belts, the complete want of structure and the presence of intersecting areas of crystalline granitoid rocks— all suggest the possibility that we have here to do witli masses which have solidified at great depths. They certainly cannot, however, be regarded as in- trusives, so that unless we regard them as great outflows, we should be forced to look upon them as the now solidified res- ervoirs from which the ordinary Keweenawan flows have come.'"* This hypothesis, though rejected by Irving, taken in connection with the facts adduced by Dr. Lawson and the further fact that there is no essential petrographic distinction between the gabbro and the diabases of the Keweenawan, but that attendant physical surroundings woiild account for all their differences, seems to explain many of the anomalies which hitherto have hung about the accepted theory of origination of the gabbro. In northeastern Minnesota the gabbro belt, along its northern border, crowds more and more toward the north. It sxTCcessively traverses the southern surface boundary of the lower Aniraikie. or Powabic quartzyte, with which it is con- spicuously interbedded, then rests against or upon large areas of the Keewatin (or Kawishiwin) greenstone; and finally is ab- solutely in contact with granites and gneisses of the Giant's range, and then falls rapidly away southward. It has been supposed that as a surface flow the gabbro spread back north- ward and thus came into contact with these older rocks in the same manner as the Keweenawan diabases have been observed to do; but it may be that this movement was a deep seated lac- colitic motion of molten rock, entering such openings in the earth's crust as the incidents of fracture and upheaval pre- sented. Such openings were liable to appear not only in the *Tlilr(l aiuiiial report of tlie direetor of tlic U, S, Oeologlcal survey, p. 12.5. the ' THE NORIAN OF THE VOKTHWEST. XXV Animikie Vjut also in the Keewatin. the lin(3 of fracture in the crust taking such a direction as the exigencies of pressure or of weakness required. Sucli fracture in Minnesota would appear to have been in a crescentic line with is concavity toward the southeast, somewhat more sharply curved than the pres- ent shore line of lake Superior. Starting from Dnluth it ran rapidly north or northeastward. Then it turned more east- wardly, at a distance of about seventy miles from the lake shore; then still more southeastward, and reached the shore again at Pigeon point; there passing under the present water surface, reappearing apparently on the northern side of Isle Royale. Such fracture line, or at least such a gabbro belt, conforms with the line of outflow-points of the Keweenawan. and with the laccolitic appearances of the trap sills in the slates of the Animikie as well as with the greatest frequency of diabase dikes. It would be necessary to allow, on this hypo- thesis, that large thicknesses of rocky strata luive been re- moved at the points where these gabbro reservoirs gathered, in order to explain their present appearance at the surface. As to the date at which this laccolitic disturbance may be supposed to have taken place, it is plain that, if it gave rise to the surface flows of trap wiiich characterize the Keweenawan, it must have been substantially coteuiporary with the Kewee- nawan. The Animikie must have been deposited prior, and per haps some of the Keweenawan. At any rate the escaping molten magma is interbedded with and constitutes some of the Keweenawan and is involved with some clastic beds. Wherever the Animikie existed it was liable to these intrusions. Where it did not exist the laccolites were formed in older rock. Where the batholitic gabbros are directly overlain, as at Carl- ton peak and at Beaver bay, by Keweenawan diabase it at first appears there was an interval of surface erosion between the in- trusion of the massive gabbro and the extrusion of the bedded diabase. The massive structureless knobs of gabbro could not thus be formed at the surface by extrusion. At such points the Animikie may have once existed. That would require that the grand epoch of this disturbance should have its commencement prior to the bulk of the Animikie; should continue through it and should have its close in the later portion of Keweenawan time. If the Animikie never existed at such points, in order to produce such unconformities there must have been eroded some older rocks in order to expose the gabbro to the later trap outflows. Such older rocks must have been some of the XXVI BULLETIN NO. VIIL Archtoan, and in that case some traces of them ought to be found at some of these interesting localities. The non-existence of volcanic debris in this period of dis- turlnince, when such deep-seated movements were taking place, allowing the transference of enormous quantities of molten rock from place to place within the crust, and the extrusion of other enormous quantities at the surface, and that, too, at a time when the presence of the sea about the vents is attested by the occurrence of interbedded clastic rocks, would certainly be an unexpected anomaly. Dr. Lawson has found nothing that indicates the existence of contemporary volcanoes in the Animikie, but such negative evidence is not quite sufficient to establish such an important principle. In this, however, while he agrees with Prof. Irving, who found no volcanic ash even In the Keweenawan, he is at variance with Foster and Whit- ney and with other observers, including the writer. It is very probable that the nature of some of the soft amygdaloids, and some of the stratiform clastic beds of the Keeweenawan have not been investigated sufficiently. It is very certain that some beds embraced between sheets of trap on the north shore of lake Superior, consisting of wholly non- consolidated materials, have the appearance of being of the nature of volcanic ash, but they have not yet been examined with care. It is difficult therefore, with this interpretation of the gabbro intrusion, to separate structurally and chronologically the later diabase extrusions of the Cupriferous from the gabbro itself, just as it is to distinguish the one from the other by any essential petrographical characters. If this hypothesis be abandoned, and the gabbro belt and the anorthosyte rocks at the localities described by Dr. Lawson, be relegated wholly to Archscan time, as suggested by him, there will arise such attendant problems that it may be found impossible to satisfy all the facts. For instance, if the gabbro (anorthosyte) dis- turbance was in ArchcJ3an time, and the diabase outflows and the Animikie laccolites, were produced in paleozoic time, and hence subsequent to the basal Taconic erosion -interval, how does it happen that the gabbro (anorthosyte) rocks are inter- bedded with the Pewabic quartzyte which is the base of the paleozoic? How also can it be explained that where the gabbro (anorthosyte) intrusions approach the Animikie slates, form- ing large bosses such as Mt. Josephine and Pigeon point, there also the diabase dykes and laccolitic sills are found to be largest and most numerous? It may reasonably be asked t THE NORIAN OF THE NORTHWEST. XXVII I also, on the l^ypothesis that the gabbro (anorthosyte) rock is of Archtuan age, how can its close petrographic alliance with the diabases be accounted for? Probably the chief obstacle to the separation of the gabbro (anorthosyte) rock from the epoch of the diabase extrusions, lies in the close areal agree- ments which they manifest. In general, the area of one is the home of the other. Wherever the gabbro occurs there is associated traprock. though tlie converse is obviously not true. The trend of the gabbro lanorthosyte) belt, whether in Wisconsin or Minnesota, conforms closely with the strike of the diabase belt. On the south side of lake Superior the gabbi'o (anorthosyte) Vjelt is to the south of the northward dipping sur- face diabase flows; on the north side of the lake it is on the north side of the southward dipping diobase tlows. In each case it allows the supposition that, on the degradation of the rocks covering the batholite while tlie reservoir was feeding the surface Hows, the uncovered batholite is separated geograph- ically from the diobase tlows by a surface interval which maij be occupied in whole or in part by some of the Animikie beds. This interval is frequently hid by drift accummulations and has rarely been studied with care. However, in any case, whether there be any intervening Animikie or not. if the in- trusion of the gabbro took place within Archtoan rocks at such places as where those rocks constituted a dry land sur- face, and the Animikie had never been deposited over them, the diabase extrusions would then lie directly upon the Archtcan, and on being uncovered the gabbro would be found intcjrbedded and otherwise in contact with the Arch;can rocks. There seems, however, to be no ready way to explain the im- mediate overlie of the diabases on the gabbro (anorthosyte ) rocks, as described at Carlton peak and eastward from Beaver bay, and as mentioned by Prof. Irving in Wisconsin, and by Poster and Whitney on Keweenaw point, except by supposing two epochs of disturbance, allowing an erosion in- terval, or one long-continued epoch with a diversified history including surface degradation at such places. Dr. Lawson's illustrations and descriptions in this bulletin are perhaj)s the strongest evidence of such direct unconformable immediate superposition of the diabase upon the anorthosyte, in situ, but it is yet to be shown that those contacts a^'e not rather upon detached masses of the anorthosyte rock. It is no un- common thing to see masses of the anorthosyte fifteen or twenty feet in diameter wholly embraced in the diabase, and XXVlll BULLETIN NO. VIIL in some instances they are two hundred feet. When numerous such masses are in juxtaposition it is plain that on exposure and glacial degradation they would present at the surface an appearance of continuous rocic in situ. The lake shore line acting on such a mass would bring out the "serrated" coast noted by Dr. Lawson, and the individual masses would appeal as knobs rising above the more rapidly destructible interven- ing diabase matrix. Such a deceptive appearance occurs at Duluth. It is the most obvious inference, based on casual examination, that at that place the melaphyrs and diabases lie immediately upon the gabbro; but between the actual outcrops of the two is an interval rather poorly exposed to observation, in which have been seen not only rounded masses of gabbro, detached from the main hill range, but also some rock of obscure characters greatly changed from its original and referable to some of the basal members of the Animikie.* This indefinite rock in other places assumes large proportions and appears as the extensive ''red rock" alternating with and cutting the gabbro and the anorthosyto rocks as at Rica's point, at Beaver bay, and at Pigeon jjoint. It has been ob- served at many places to pass into a sedimentary rock. It is frequently i)lainly a conglomerate originally. This belt of changed Animikie appears to run with the gabbro from Duluth to Pigeon point. At, the latter -p\-a>cc Prof. W. S. Bayley has made a special study of the contact phenomena of the gabbro on the slates and quartzytes and his conclusionst lately piiblished confirm those of Richard Owen and J. G. Norwood,]; and of the writer published in many jolaces. to the effect that the red rocks of the Cupriferous, including the quartz porphyries and red syenites specially, are modified conditions of sedimentary rocks. The persistence of this modified belt, giving rise to soda granites and augite syenites, or to "quartz keratophyres," referable to pro-existing (proba- bly) Animikie strata, precludes the non-existence of the orig inal Animikie strata throughout any wide extent between Duluth and Pigeon point, and looks toward their probable future discovery, perhaps in some of these disguised forms, in the vicinity of Beaver bay, and hence to the probably detached condition of the anorthosyte rocks on which the diabase flows are seen to lie at that place. ^— .^^^^— ^^^— _— I *Niiitli uiiiiuiil report, p. 11. under "1 D;" p. V2, iiiidor "7:" p. 1", under "42;' teutli report, i)p. 107-1(H>. under ••807" and "808." + Am. .Tour. Scl. XXXIX. '^73, 1890. * Report of ti OeoloKlcal Survey of Vv'isconsiii, Iowa and Minnesota. D. D. Owen. THE NOUIAN OF THE NORTHWEST. XXTX n numerous n exposure surface an shore line ■ated" coast luld api)eai le interven- 3 occurs at i on casual I i abases lie al outcrops bservation, of gabbro, e rock of iginal and Animilfie.* )roportions g with and at Rica's s been ob- ock. It is is belt of )bro from rof. W. S. nomena of )nclusions+ and J. G. es, to the udi ng the modified ! of this syenites, g (proba- the orig between probable ed forms, probably iiich the under "412;' The existence of this extensive "red formation." so closely an attendant upon the gabbro belt, is suggestive of still another hypothesis. Brielly the characters of this rock may be sum- marized. It is. speaking broadly, a rock of orthoclase, horn- blende and quartz. The predominating orthoclase is sometimes so coarsely crystalline as to be macroscopically evident, but is more frequently so indefinite as to show no crystalline texture. Vjecoming felsitic. The coarser crystallization is found in the large bosses which sometimes make mounlain-lilce hills and ranges. The finer sorts are seen as fjuartz-porphyries. felsytes. intersheeted with diabases or cutting them, and as veins in the gabbro. The feldspar is not wholly orthoclase. but is some- times seen to be a striated plagioclase, which Prof. Irving considered oligoclase. Like all the other minerals the ortho- clase is reddened by abundant ferric oxide. This has been con- sidered a secondary product of alteration, but it is more likely to have been originally in the rock from which the "red rock" was derived. The ferro-magnesian mineral is usually horn- blende, but Pi'of. Irving found that very often it is augitic. He inferred that the hornblende is wholly derived by alteration from original augite. It is very certain that it is sometimes chloritized. The quartz is the most interesting of all the min- erals, as it assumes positions and forms which indicate its origin. In some cases it prevails over all the other ingredients, making a quartzose rock, which should receive ratlier the name gneiss. Prof. Bayley gives it on Pigeon point the name "quartz-keratophyre." Those grains of (juartz which were in the original rock have undergone some molecular changes, but usually not enough, especially in the finer grained portions, to destroy their clastic characters. The "club-shaped" quartzes seem in the red augito-syenites associated with the gabbro at Duluth, were pronounced to be secondary quartz by Prof. Irving.* and that which had been so completely fused as to crystallize in- dependently amongst the orthoclases, was considered by him as the original quartz. The reverse may be true, the original fragm.ental quartz grains having been drawn out into the forms which are illustrated, and some of them rearranged as to out- ward form, and all of them as to crystallographic directions. When complete fusion was superimposed on the sedimentary mass, the quartz grains were blended in one, and on cooling were forced to accommodate themselves to the adjoining orthoclases. In the felsytes the quartzes are sometimes in D. D. Owen. *See parlloiiliirly Moii.V, V. S. Oeol Sur., PI. XlVaiid the acconipanyiiig Uescripiloii. XXX BULLETIN NO. VJII. semiroundod grains and sometimes are doubly-terminated crystals. These rocks show almost every conceivable manner of association with the gabbro. They cut it in dikes, both per- pendicular and inclined, and they swell out in large patches, surrounded by the gabbro. They underlie and overlie large areas of gabbro, and both are cut by the later dikes of diabase. They are cut olT by perpendicular bosses of the gabbro, some- times having a perpendicular or inclined line of close contact without blending, and sometimes the elements of the two rocks are united as by a mixture in one common magma, making them, apparently, the so-called orthoclase gabbro, as seen at Duluth. Considering the diabases of the early portion of the Keweenawan as derivatives from the gabbro batholites, these rocks when cutting and alternating with the diabases, may in like manner be considered as cotemporary derivatives of large batholites or other masses of red rock ready to enter any open- ings in the surrounding crust where they could tind relief from thecrustal pressure. As pebbles, they are more durable than those of gabbro, and they constitute large beds of conglomer- ate in the base of the copper-bearing rocks. They are found as transported blocks or boulders in association with trans- ported blocks and boulders of the gabbro inclosed in the later traps, as already fully noted at Beaver bay, and about Encamp- ment island. Chemical Analyses of the Red Rock. — For purposes of further comparison the following table is compiled of such analyses as are at hand of this "red rock" formation: I. From the west bluff at the entrance to Beaver bay harbor. Analysis by J. A. Dodge. Building stones of Minnesota. Vol. 1, of the final report, p. 198. Survey No. 124. II. Rice point, red granite; by Prof. J. A. Dodge. Tenth annual report; p. 204, Sur. No. 1 B. III. Another analysis of the same as II, Prof. J. A. Dodge. Thirteenth annual report, p. 100 (No. 148). IV. Finely crystalline brown syenite; Duluth, J. A. Dodge. Geol. Sur. No. 7. Thirteenth report, p. 100 (No. 149). V. Fine grained, reddish brown rock; Duluth, J. A. Dodge. No. 19 of the Geo. Sur. series. Thirteenth annual report, p. 100 (No. 150). VI. "Streamed,"' light red, with translucent laminations and specks. London (near Duluth). J. A. Dodge, Thir- teenth annual report p. 100 (No. 152). Geol. Sur. No. 68. I THE NOUIAN OF THE NOUTHVVEST. XXXI VII. Brick red, rather fragile, apparently j?ritty and sub- crystalline. J. A. Dodf?e. About two miles east of the mouth of Passabika (Lester) river, near Duluth. Thirteenth annual report, p. 100 (No. IJS). VIII. Purplish rod ufranite from the west bluff at the entrance to Beaver bay. J. A. Dodge. Another analysis of the same rock as No. I. Geo. Sur. No. 124. Thirteenth annual report, p. 100 (No. 155). IX. Grayish-red folsitic rock, in metamorphic cpiartzyte or quartzose interlaminations of ••streams" of siliceous matter. Dodge. Thirteenth annual report, p. 100 (No. l^S). the field taken to bo a shale, rather slaty, with J. A. Geol. Sur. No. 127. The analysis shows this rock to bo nearly iden- tical with VIII, with which it probably has a connection. They outcrop near adjacent, at the west side of Beaver bay. X. Red granite from the third island below Beaver bay. J. A. Dodge. Thirteenth annual report, p. 100 (No. 157). Geol. Sur. No. 184. XI. Rock of the bulk of the Great Palisades, a red quartz porphyry. J. A. Dodge. Thirteenth annual report, p. 100 (No. 158). Geol. Sur. No. 139. XII. Red. laminated, or •streamed," at the base of the Great Palisades. J. A. Dodge. Thirteenth annual report, p. 100 (No. 159). Geol. Sur. No. 140. XIII. The red rock at Grand Marais; furnishes the pel^bles of the beach. J. A. Dodge. Thirteenth annual report, p. 100 (No. 162). Geol. Sur. No. 203. XIV. Red rock from the first island northwest of Belle Rose island, south of Pigeon point. J. A. Dodge. Thirteenth annual report, p. 100 (No. 164). Geol. Sur. No. 285. This seems to be the augite syenite examined by Prof. Irving, from Brick island. XV. Analysis of the powder of seven specimens of the granular varieties from Pigeon point; W. P. Hillebrand. Re- ported by Prof. W. S. Bay ley. Am. Jour. Sci. (3). xxxvii, 59. XVI. Powder of three of the quartz porphyries from Pigeon point; W. F. Hillebrand. Reported by Prof. W. S. Bayley. (A. J. Sci.) XVII. For purposes of comparison this analysis was made of one of the associated clastic rocks. It is from a pinkish quartzyte at the head of Wauswaugoning bay, near Pigeon point, a rock which is found in immediate contact with the red rock and with the gabbro on Pigeon point. J. A, Dodge; thir- teenth annual report, p. 100, (No. 163.) Geol. Sur. Series 262^ XXXll UULLETFN NO. VIII. XVIII. This analy.sis was also made for comparison. It is of a rod sandstone, or quartzyto, ftntv^i'ainod. from tlio north sido of Siskiwit point. Isle Royal, formerly quarried for build- ing stone. J. A. Dodge; thirteenth rei^orl, p. 100 (No. 10')). Cieol, Sur. Series 555. An inspection of this table of analyses will convince anyone not only that the red rock series is a geologic unit, extending from Dululh to Pigeon point, thus agroeeing with the result derived from field evidence, many times reported, and with microscoiiical examination, but that it is referable to a change of sedimentary rocks immediately adjoining th(5 gabbro bosses, and in that also agreeing with the results of extended field studies, the most thorough of wliich are those of Prof. W. S. Bayley on Pigeon point. It is therefore a legitimate inference that where this red rock is most abundant, there the gabbro acted most powerfully and profoundly on the strata concerned. The Animikie, therefore, must have been fused through that whole bolt where this red rock prevails — that is, in all that region north and northeast- ward from Grand Marais, and also westward toward Beaver Bay, and finally to the vicinity of Duluth, through which the Animikie rocks ought to be found, unless some such unusual event supervened to cause their disappearance by converting them into this pu/zling red ro"k. It is not necessary here to dwell on the importance of such a result. The writer, as he pursues this review, becomes im- pressed with the probable correctnesss of this last hypothesis, and is constrained to adopt it as a working hypothesis for future research. Lithologlsts have looked at the augite syenytes from the side of (U'iginal fusion and eruption, and all their microscopic characters have been interpreted and defined in terms of modern petrography, which hardly yet may it be said to have recognized any other source for crystalline rocks. Hence in reading the descriptions of these rocks (as those by Irving) many ideas of genesis and alteration must be reversed. It is also difficult to distinguish fact from hypothetical presump- tion. Facts and presumption are expressed with equal posi- tiveness— nay. sometimes presumption is made more positive than fact by the addition of adverbs and phrases that are intended to highten the author's assurance and exclude the reader's possible doubt, as if there were no possibility of error in the initial data. TIIK NdUIAN OF THK NORTFiWKST. X X XIII f M : 5^- S 'd : 'i? ' '5 5 'A ■ 01 : TJ 1 r^ • O -f — o a -__ — ; : 1 ^ wm Sf • a? Iff : J? : ?i > ao ; a: — CI . =■ : '-^ o — : " : : 8 n 1 8 S 3 S g 8 !£ 2i 2.' ro i; . -s 8 . ! '^^ > X 5 3 :S : ■J f > i 5 - iO t- 3: I- rt ^ •-- «. > : ?- fl ?5 r, CO 1- 3 o •f X ; t Cl — o «-) 01 § .^ : i? ,^ g " 00 * M ""i"" • rt ro 3 - o 01 01 — ; , 'i> >H 3 r- s • li? : s M i • — ro r w O — - 00 d "r ' -* « TJ ■!>1 ": f, l.O 'W~ S : . "^ >1 1 ;o 1 t- •M .~: o • o ~o -it* • * s; f s ro « » -C » lO 3 35 ~i"l^ x' r- z: '^ ^ : ^S'^^ 1 1 - 3-. •-^ • ' 2 1 ="- >< ! ?: n 9. ^ : SJ : : "7"^'^ 1 - ZJ "^ ^ : o O 01 o 01 • 1- 3i O — i~. ' "■ •^ 'M 1- s . M 5 o 1- 01 ao ! " JJ O O 1-^ • p-^ 0^ ;?3 1 Ol • • ^ HH 1 fg 5 ^ ?1 . 31 > 1 '« O -t c o O t.t ro d "d" -V oi 0^1 o JO iM — 'M CO "."J CI ■;i -M d ■ 1-^ o o 1- oi 3 d • • 1^ . S : S S :;? : (N o 00 S : S6 i* 1 - in 2 — -t ! : - : '-' o '- ; " ; ; oo 1— 1 W in .r =: Tt< a: lO re as in 1-0 ^ : S : : 1 Ol •-4 CO o : "* -^ « 00 -f o ^' • rl ^ « M c-. i: ro x- 55 -p '.-5 o CO 53 ■ '• 1— f^ Jc ; w 1- 1) 01 — ^ 01 I— t ■ 8 X' S 2 -/J ?p § -f Ol ao i - ^ o — -M r CO CI - --^— o '- - : '^ ; ; 1 ! SC : c-i cq 01 S5 i;3 • ' 8 " I— -M ;c 01 o '-' 01 jS 1 ! c d" o' c C c o O o ^^ cZ ? ^' -J' > <1 i, * c a ^ •A Si J' r, '-J \X.\IV ML'I.I-KTIN NO. VIII. ir lh«!st) rod rocks ai'o rot'urabU' to tlio cuiis*! Iiort! suppo.swl. vi/: a Hocondary prodiu^t l)y fusion of sodiriK^ntary rocks, it will bo nt'ccs.sary to inako profound cluui^'os in Iho inotliods and masoning' of some iiiirrosco])ical students. Thoy must begin thoii* study from tlio opposite etui of the i)roc(!ss. The inici'o s(tope must reverse its line of i)ursuit. and s - .ii; 1 INTRODUCTION. The Minnesota coast of lake Superior between Duluth and Grand Portage was mapped and described by Irving, *as exclu- sively occupied by formations of the Keweenian (Keweenawan) series, and his discussion of the geology of the coast is cer- tainly the most sjctematic which has up to the present ap- peared. State Geologist Winchell had earlier made careful examinations along the same line of outcrop, and had placed on record a host of detailed observations fon the rocks of the coast, without, at that time expressing any final conclusions as to their stratigraphy and general relations. Quite recently, how- ever, we have an expression of Prof. Winchell's views of the geology of the coast of Minnesota in the form of a map issued by the State Survey. jProm this map we gather that Prof. Winchell is in accord with Irving as to the Keweenian age of that portion of the coast which lies between Duluth and Gi'and Marais excepting two small areas at Beaver bay and the vicin- ity of the Palisades; but differs from him in classing as "Norian" the rocks at Duluth and the most of those between Grand Marais and Grand Portage, together with those of the small areas above mentioned at Beaver bay and the Palisades. This correlation of the Duluth gabbro and the black gabbros east of Grand Marais, together with the red granites, quartzi)or- phyries and red felsytes of the coast, as a set of formations equivalent to the Norian is a suggestion which the writer is not prepared to follow. As it is, however, simply an expres- sion of opinion, not yet supported by the publication of the facts and arguments which influenced Prof. Winchell to its adoption, it calls for no criticism. But it is the accordance of Winchell and Irving rather than their differences of opinion to which the writer desires to direct attention, in order to make clear the views which obtain in the literature of the subject, as to the absence on this coast of •(^opper-heai'lriR Koeks of Lake Superior.— MunoKi'iiph V. U. S. G. S. Cup. VII. tGeol. ami Nat. Hist. Survey of MiniiCHUla Ulli and lOtb Annual llepurts. WJeoloKlcal map of the Iron Regions of Minnesota by N. H. Wincliell, and H. V Winoheli, 1890. Auuompauyiug "Iron Ores of Minnesota." BULLETIN NO. VIU. rocks of any geological horizon other than the Kewecaian, or than those classed by Winchell as Norian.|| The preposition which the writer lays down in this paper is that he has dis- covered on the Minnesota coast a geological formation of strongly marked individuality, which is distinctly older than the Keweonian, and is separated from it by a period of pro- found erosion. This formation is in no way connected with the rocks classed by Winchell as Norian.|| The rock of which this formation is composed has been described by both Winch- ell* and Ix'ving.f and was also observed by Norwood I Nor- wood and Winchell first called the rock "feldspar rock" and the latter named the feldspar of which it is composed labra- dorite, subsequently referring to the rock as "labradorite rock." Irving after a seemingly thorough examination§ of the feld- spar determined it to be anorthite and referred to the forma- tion as "anorthite rock." None of these observers seems to have apj^reciated the significance of the field evidence as to the true relations of the formation to the Keweenian, and in- deed no discriminative effort of any kind has been made to separate it from the prevailing series of the coast. It is the purpose of this paper to set forth those relations, and to establish, by clear and unequivocal evidence, the ex- istence on this coast of a new geological horizon of much interest. The fact that the rock has been \ari()usly named by the only investigators who have published descriptions of it renders it necessary that the evidence of its true petrographical char- acter should be first passed in review, particularly as the rock is in itself an interesting type whose importance is not yet fully recognized in petrographical literature. Following the petrographical notes, will be given a brief account of the dis- tribution of the formation along the coast, and this will be fol- lowed by a discussion of its relations to the Keweenian, and of its probable correlation as a pre Keweenian horizon. PETKOQKAPHICAL CHARACTERS. Irving's dcscripflon: — Irving's summary statement of the pet- rographical character of this formation is as follows: "At several points on the north or Minnesota shore of lake Super- ♦Nlnth Annual Report, I'p. 20, HO. Tenth Annual Report, p. ;«). +c;()l)|)er-»eariii(; rocks of Lake Superior, p. riD-til ; •j:!8-440. JUoportof II Oeolottical .Survey of Wlsi-onsiu. lowii and Minnesotii, by I). D. tUven. 1852. pp. ;tOO, 301, 380. nXliese are .some of tlie rocks ol«s.sed by Wiueliell as Nortan. LN. H. W.l HLoc. ult. THE AN0RTH0SYTF:S of MINNESOTA. 3 jraian, or •c position e has dis- nation of )lder than )d of pro- cted with : of which th Winch- )d t Nor- rock" and ied labra- i-ite rock." the feld- the forma- s seems to ence as to an, and in- n made to 3 relations, 36, the ex- 1 of much jy the only . renders it hical char- is the rock is not yet lowing the of the dis- will be fol- lian, and of of the pet- ows: "At ake Super- by 1). D. Owen. ior. between the mouth of Splitrock river and the Great Pali- sades, and again in the high point near the mouth of Temper- ance river, known as Carlton peak, are to be seen exposures of a very coarse, light gray to colorless or white rock, occas- ionally with a faint greenish tinge. This is seen in thin sec- tions' to be composed exclusively or nearly so of anorthite feldspar. Often there is no other mineral present except in exceedingly minute inclusions and these are very sparse. In one section a few gi'ains of altered olivine were noticed within the anorthite, and in two or three a little augite between the feldspar grains. The feldspar appears in every case to be anorthite. In no section did it show the peculiar arrangement of needle like inclusions met in Euroiiean gabbros, and so com mon in the coarse gabbros of lake Superior, to which this rock is very nearly related." Supplementary description: — This description requires to be both supplemented and modified. One of the most remarkable features of the rock is its extreme freshness. The constituent feldspar of the rock is never found in a decomposed condition, but presents on the contrary uniformly, in all parts of the for- mation, perfectly glassy and brilliant appearance. Another remarkable characteristic of the rock as a formation is its massiveness and lack of structural planes of any kind ex- cept under some abnormal circumstances which will be more fully mentioned in the sequel. Even jointage may be said to be entirely absent. Occasionally the formation is traversed by one or moi-e fissures locally, but these follow no law of direc- tion and cannot be regarded as true jointage. There are few of the massive roclcs of the lake Superior region so free from structural planes. There is no flow or gneissic structure, and the rock seems nowhere to have been subjected to forces which would tend to deform the mass, render it schistose or in any way induce the development of secondary structures of a me- chanical nature. The color of the weathered surface of the rock is prevail- ingly white or yellowish white. The surfaces are usually smooth in general, but minutely rough, showing often the solv- ent action of meteoric waters in the same way that marbles and limestones do. though in a less marked degree. The weath- ered layer is very thin, and the fresh glassy crystals are ap- parent through it. The color of the rock on fractured surfaces is usually as Irving described it, but there are also darker, greenish-gray facies. BULLETIN NO. VIIL The texture is prevailingly coarse, the cleavage faces on the conslitutent feldspar being frequently half an ipch in diameter (occasionally an inch) and ranging from that to one quarter of an inch. In some localities it talces ou a finer texture, and would then be designated a medium grained rock. The structure of the rock is eminently allotriomorphic- granular throughout the entire formation. No porphyritic crystals are anywhere ob servable, and under the microscope no suggeston of idiomorph- ic forms could be detected. The Const'duenf Feldspar — The rock is, as former observers have noted, composed almost wholly of a plagioclase feldspar. In the majority of thin sections no other mineral, except in the form of minute interpositions, is present, and in the few sections in which a ferro-magnesian silicate may be observed, it plays only an accessory r(')le. This plagioclase is not, however, as Irving af- firmed, always anorthite. Neither is it, as Winchell believed, always labradorite. In an attempt to reconcile the conflicting statements of these two geologists, the writer has discovered that some portions of the formation accord with Irving's deter- mination and other portions with Winchell's; that both were probably right as regards the material which they subjected to examination, but that both erred in generalizing as to the uni- form character of the feldspar throughout the formation. The determination of the feldspar by optical and chemical exami- nation is rendered comparatively easy by its perfectly fresh condition, its pronounced cleavage, the constant presence of strongly marked, polysynthetic twinning, and by the ease with which material free from interpositions can be selected for , analysis. These interpositions are not uniformly distributed, and owing to the glassy transparent character of the crystals, it is a simple matter to separate among small fragraments, pieces which are perfectly clear and vitreous from those which are more or less charged with inclusions. With such favorable material a portion of the rock formation was definitely de- termined to be composed of labradorite. ' Optical Measurements: — For optical observations a number of thin sections were prepared strictly parallel to the basal pina- coid, and to the brachy pinacoid, and numbers of readings were taken for the extinction of adjacent twin lamellae. The follow- ing results were thus obtained, the figures given being the mean extinction for the two sets of slightly discordant readings on either side of the cross hair of the microscope. The cleav- % THE ANORTHOSYTES OF MINNESOTA. es on the diameter ,rter of an ,nd would iicture of ^hoilt the vhere ob iomorph- •vers have ir. In the the form actions in )lays only Irving af • believed, jonflicting liscovered ]g's deter- both were bjected to o the uni- tion. The ;al exami- ctly fresh esence of ease with ected for , stributed, crystals, graments, ose which favorable litely de- lumber of asal pina- ings were he follow - jeing the readings 'he cleav- age fragments selected were taken from material collected on the shore near Encampment island and from Carlton peak. Extinction Angle Extinction Angle i 4 onO. P. (001) on au P ^ (010) 21° 17 21° 27 21° 27' 22° 21' 23° 33' 25^ 00' 8° 4r 9° 00' . 9° 02' 9° 07' 9° 08' 9° 09' 9° 12' 9° 18' 9^ 22' 9° 55' 10° 49' 10^ 57' 11° 00' These results locate the feldspar in the plagioclase series somewhere between Ab3 An^ and Abj Auj, and it may there- fore without hesitation be designated a labradorite. Coujirhiatory tests : — In order to contii'm this conclusion the material from near Encampment island was subjected to further tests. A calculation based upon the extinction angles above recorded, showed that the composition of the feldspar was about Ab, An.j and the chemical composition was predicted for it as it is given in column I of the table of analyses. Care- fully selected material was then analysed (by W. C. Blasdale. Fellow in Chemistry in the University of California) and the results obtained are given in the adjacent column, II. This analysis again establishes the labradorite character of the feldspar. Its specific gi'avity, as determined on glass- clear fragments with the aid of Klein's solution, is '2.701' which is in entire accordance with its optical and chemical character. The powdered mineral, moreover, is insoluble after prolonged boiling in hydrochloric acid, the powder being in appar- ently the same condition at the close of the operation as at the beginning. There is thus no doubt whatever as to the mineral being labradorite. Bock composed of Labradorite : — No other feldspar than the one examined can be detected in the rock. Indeed the material selected for analysis was not from a single crystal but from crystals in various parts of the hand specimen. It is there- fore believed that the whole rock at this place is composed of labradorite feldspar and that no anorthite is present. This € BULLETIN NO. VIH. view is sustained by the results obtained for the specific gravity of the rock as a whole. Three fragments of the rock taken at random, and ranging in weight from three grammes to sixteen grammes were used. The three results were 2.702. 2.704 and 2.706. The mean of these, viz : 2.704. varies so little from that of pure labradorite, that, in the total absence of any lighter non-feldspathic minerals, it may be safely inferred that the rock is composed practically of one feldspar, viz: labra- dorite. A bulk analysis of the rock, made upon fragments taken at random, by Mr. Chas. Palache, Fellow in Mineralogy in the University of California, is given in column III of the table, and also demonstrates the fact that the rock as a whole has essentially the composition of labradorite. Roc/i compofied of Anorthite: — ^A specimen from another local - y, viz: from the cave to the east of Split-rock point, was broken into small fragments and pure transparent vitreous fek^spw was again selected for analysis. This was submitted !o i'l >'. J. A. Dodge of the University of Minnesota, and the resuUs of iiis analysis are given in column IV of the table. This analysis shows that the feldspar at this locality, though rather rich in silica, is essentially anorthite, the alkalies being very subordinate in amount, and this confirms the correct- ness of Irving's optical determinations of the feldspar of this rock from certain localities on this coast. Irving's analysis* of a specimen from a point two miles below Beaver Bay is given in column V, but the relative proportions of lime and soda, as well as the specific gravity, suggest that the feldspar examined is bytownice rather than anorthite. Chemical Armlyses: — The following table gives the result of a few chemical analyses which have been made of these rocks and of their constituent feldspars, f together with two analyses quoted for comparison: SiOg 53.01 AlaO., ' 30.04 Fe,u, I FeO CaO 12.37 MgO 1 IStiaO 4.56 KsO HaO Total 99.i)8 SpG II. III. IV. 51.;}0 I 47.40 ; ")1.4n 31.46 12 20 "'5.'33" '"'.ib' "i.hiyi 29.74 1.94 13. ,30 .57 4.99 1.56 1.64 101.14 31 94 Trace 14.31 .27 .85 .21 .68 99.71 2.7041 2.709 V. 31.56 VI. VII. 47.30 49.155 31.50 I 29.620 1.85 I 1.152 2.29 15.39 .27 2.52 .37 .40 100.05 2.70 14.88 .93 1.22 .38 1.80 99.86 15.309 .911 2.914 .695 .730 100.486 *Copper-l)eaflnK Rocks. Monograph V, U. H. G. S., p. 438. + The thairk.s of the writer aro hero teiiden>d to Prof. D(Kl){e and to 7«Iessrs. Blasdale and Palache for kind assistance In tuakin;; analyses. I THE A.NORTHOSYTKS OF MINNESOTA. ,he specitie of the rock e grammes were 2.702. ies so little 3nce of any iferred that , viz: labra- fragments Mineralogy I III of the as a whole ther local - point, was nt vitreous i submitted ta, and the the table. Lty, though :alies being he correct- par of this analysis* ver Bay is lime and le feldspar e result of lese rocks o analyses ^I. VII. .ao 49. 155 .50 29.620 .85 1.152 .88 15.309 .93 .911 .22 2.914 .38 .695 .80 .730 .86 100.486 39srs. Bliiadale I. Theoretical composition of labradorite Ab^ An., the latter being the formula calculated for the constituent feldspar of the rock from near Encampment island, on the basis of optical measurements. II. Analysis of the same feldspar. III. Bulk analysis of tlie rock from near Encampment island. IV. Analysis of constituent feldspar of the rock from cove east of Split rock ]K)int. V. Analysis of the rock from point two miles below Beaver Bay as given by Irving. Copper Bearing Kocks, page 438. VI and VII. Two Analyses of bytownite, ([uoted by Teall, British Petrography. i)age 146. The name AnorthoHyU' : — It is thus clear that the mineral ogical composition of the rock is not sti'ictly uniform and that both the names which have been applied to it as a rock formation are un- fortunate. Neither •' anorthite rock " nor " labradorite rock" is a correct designation for the rock as a whole; and if a more satisfactory and comprehensive designation can be found, both of these terms should be dropped. The essential and constant feature of the rock is that it is almost exclusively composed of an allotriomorphic granular aggregate of basic i)lagioclase. Such rocks are not unlcnown to the writer. Large areas of them occur in eastern Canada, and he has met them in the field at Narodal in Norway, and in parts of northern New Jersey, and in the Rainy Lake region. Such rocks have not yet. however, received a satisfactory place in petrograhical classifications. While considering the question of a designa- tion for such rocks, the writer had occasion to consult with Prof. Frank Adams of McGill College, Montreal, who, from his long study of similar rocks in the so-called Norian series of Quebec, is probably our best authority in this branch of petrographical science. In this correspondence Prof. Adams informs the writer that he has now in the x^ress a memoir on just such rocks as form the subject of this paper, treating them historically, petrographically, geologically and compara- tively. In this memoir Prof. Adams will retain the old name " anorthosite" for the class of granular rocks composed of plagioclase to the i)ractical exclusion of the ferromagnesian silicates. Such rocks are regarded by him as being at one end of the gabbro series, while at the other end are those granular rocks composed of ferromagnesian silicates almost to the exclusion of the ijlagioclases. The term anorthosyte 1 8 BULLETIN NO. VIIL is therefore here adopted for the plagioclase rocks of the Minnesota coast, in accordance with the usage which will certainly be established by the publication of Prof. Adams' memoir. Accessory Const Uuents of the Anorthosyte:— The only original mineral which can be discovered in the slides as a constituent of the granular aggregate, is a faint violet brown monoclinic pyroxene. It shows none of the lamellar structure of diallage, and is evidently a feebly ferriferous augite. This augite, as has been stated, plays a very subordinate role, and is found in two modes of occurrence which are not sharply separable: (1) As minute irregularly shaped patches, usually some- what triangular, filling a few interstital places between the large grains of plagioclase. These particles are usually of uniform orientation throughout and consist of a single indi- vidual crystal. (2) As more or less rounded or bleb-like in- clusions within the plagioclase crystals. In the latter case the plagioclase sometimes shows, by its undulatory extinction in polarized light, an area of molecular tension encircling the crystal of augite. In both occurrences the augite may be charged with magnetite dust, but is often seen free from such inclusions. In some slides the augite is perfectly fresh and shows no decomposition whatever. In other cases there is observable a fibrous structure occupying a peripheral zone of varying width, the fibers being all parallel. In still other cases the augite has become greenish, due to the presence of ftocculent chlorite, and in others the whole of the augite has passed over into a nest of scaly chlorite with, perhaps, some serpentine. In one instance a shred of green hornblende was observed on the periphery of a partially decomposed grain of augite, and is probably also a secondary product. Interpositions: — The inclusions or interpositions in the plagio- clase are of three general kinds: (1) Original mineral in- clusion arranged in plates or rods parallel to definite crystal - lographic planes. (2) Original liquid and dust-like inclusions arranged in irregularly curving planes without reference to the crystal structure. (3) Secondary inclusions of red iron oxide in minute specks, arranged peripherally to the plagio- clase or along the cracks which occasionally traverse it. The interpositions of the first class illustrate in a very strik- ing way the same phenomenon that has been described by Judd as due to the process of Schillerization. ( See plate II, Fig. 1). The mineral plates when seen on edge are of nearly .0- ks of the i^hich will •f. Adams' ^ original onstituent nonoclinic f diallage, augite, as i found in rable : i,lly some- tween the isually of ttgle indi- eb-like in- r case the inction in rcling the i may be rom such fresh and there is al zone of till other esence of bugite has aps, some Lende was grain of le plagio- Lueral in- 3 crystal- nclusions jrence to red iron 3 plagio- it. 3ry strik- xibed by I plate II, )f nearly '''"^■'■'••- "• '•■'<■■ '• ^lirr..|,li..l.,^u,,,|, ,,t -..,•11,-11 of ;umrt'ai- lo th.> left in the i 1 1 iisl rat ion. ( ill i| . \ \ li \ \ I •! I I- I M K'. >I I \.\ I il III I I \ \m •« I 'I \ II II I THE ANORTHOSYTES OF MINNESOTA. -* uniform thickness but of varying ien^th. They iu-»( not present in all slides, but when they occur they ure prominent features of the section. They aj»pear in all cases to lie parallel to the plane of the brachypinacoid, and thus between crossed nicols they a))pear in soctifnis transvtu'se to x P i as hd in the same slide with the augite which occurs intei'stitially be- tween the feldspars, and no essential difference can be detected, the color and refractive power being the same. In many cases, however, a single mineral {ilate appears to be mad«! n\) of an aggregate of minute; granules of augitc. The plates are also frequently charged with granules of magnetite. The relation of the plates to the plagioilaso is such as to suggest that they are original inclusions, the feldspar being quite fresh and the contact of the plates with plagioclase, when studied with high powers, being irregular in detail. It thus appears from their mineralogical character, and from the mode of their iiiclosure that they ditfer from schillorization products; and that they are original inclusions although presenting precisely the same appearance under the microscope as the schillerized minerals described by Judd. The inclusions of the second class are abundant but usually very minute. They may be observed frequently congregated in crescent-shaped areas and in these cases the inclusions com- monly are pear-shaped. Bubbles may rarely be seen but they are not apparently mobile. In almost all cases where these in- clusions appear to traverse a crystal in bead like rows, these rows may. by careful focusing, be seen to be but the traces of planes. These planes are usually irregularly curved and in- tersect one another in all directions. The inclusions of the third class are abundant only locally, and then they give a faint reddish tinge to the rock as may be seen in sjiecimens from Carlton peak. They appear clearly to owe their origin to percolating media, since they are formed only on the periphery of the feldspar between neighboring grains, and along the cracks which traverse them. The color is a bright red and the inclusion is usually a minute irregular patch. 10 BULLETIN NO. VIIL r, I DISTRIBUTION AND MODE OK OCCUHUENCE OK THE AN0RTH08YTE. Two Modes of Occurrence: — Anorthosyte has been observed by tlie writer at several localities on the Minnesota coast between Encampment island and Carlton ptsak, a distance of 46 miles. Its occurrence seems to be limited to the middle third of the <'oast. The re) In the ilocks im- nd Irving is found, localities, of occur- cvn in this ions. down the which the 3ut half a Pp. 53, R. ry numer- ^gdaloidal lian flows, ■al project c in which ea of the iers were est cross- lany small inches in detached itter were L with dia- and their vith knife- Lilders are p grooves. 3neral sur- nstructive of glacier y ^ I I:. rS; 1 is THE ANOliTHOSYTES OF MINNESOTA. 11 Vicinity of Split-rock point: — The second locality where the anorthosyte forms a prominent feature of the geology of the coast, is at Split-rock point, and at the bottom of the small cove immediately below the point. Here it is found in both the modes of occurrence above mentioned. The extremity of Split- rock point presents a very remarkable aspect. It is a sheer cliff rising vertically' fifty or sixty feet above the surface of the water. This wall of rock affords a clean section of a gi^eat breccia. The volcanic rock which forms the point, a diaba.se- porphyryte, is studded with innumerable boulders and angular blocks of anorthosyte. The matrix being very dark and the included blocks whitish or yellowish, the contrast is pro- nounced. The size of the bloclcs ranges from a few inches to several feet in diameter. They are in some places congregated together and in others they lie isolated in the matrix. Around the point to the east we find the anortho.syte in place on both sides of the cove. On the west side a mass a hundi-ed yards or more in extent is exposed, and the diabase-porphyry te is clearly seen, both to traverse it in sharply defined dykes, and to mantle its upper surface in the form of a flow. Here also may be observed a structural and mineralogical diiforon- tiation of the anorthosyte, which is apparently quite local in its character, and which has been observed by the writer only at one other exposure. This ditTerentiation is apparent in the form of dark bands, which traverse the mass and give it a bed- ded aspect, as is exhibited in the illustration. Plate II, Fig. 2. These bands are usually only a few inches thick, but may some- times be as much as a foot across. They have at this locality a characteristic jagged appeai-ance, owing to the fact that the band has frequently no simple line of demarkation separating it from the normal anorthosyte, but is interlocked with it by means of sharp, wedge-like prolongations or tongues, which are approximately parallel to the trend of the band. The dip of the planes of the banding is to the north-east at an angle of about sixty to seventy live degrees. The banded rock is tra- versed by three small dykes of the diabase-porphyryte which have a nearly vertical attitude, and H'hicli are easily discrimi- nated from the bands on the ground, although they do not ap- pear very differently in the photograph from which the plate has been prepared. These bands are mineralogically different from the normal character of the main mass of the anorthosyte in having a considerable proportion of a dark ferro-magnesian constituent ( augite) which is more or less decom]iosed. Under 12 BULLETIN NO. VIIL the microiif;ope, the rock which composes the bands is not otherwise d liferent from the normal anortlisyte, and the plagio- clase appears to be in no way affected cliemically by the de- composition of the augite, although the rocli exhibits a tend- ency to mechanical disintegration by the separation of the feld- spar grains. On the other side of the cove, secton 6, township 54, range 8, the anorthsyte is exposed in much greater mass. Here it emerges from beneath the Keweenian eruptives and stands out as a bold, bare ridge v/ith somewhat rounded outline, rising to an elevation of about 200 feet. The section here presented is shown diagramatically in the sketch Pig. 1. The base of the anorthosyte ridge is lianked to the southeast by a mass of red, 'ine grained amygdaloldal fieldspar-porphyry; and this in turn ■s flanked lake ward by an amygdaloldal diabase porphyry te, In A'hich lies imbedded a boulder of the anorthosyte. Irving's views on the occurrence at Split-rock: — Irving's general note on the occurrence in the vicinity of Split rock point indi- cates that he regarded the anorthosyte as an eruptive mass, although his own observations are in part quite inconsistent with this view. He says:* "Near the middle of S. E. ^ of Sec. 5, township 54, range 8 east, this gabbro (one of the Keweenian flows) Is interrupted by a vertically placed mass of excessively coarse- grained anor trite rock [anorthosyte]. The cutting mass is from 50 to 75 feet wide and bears north and south. It shows on both sides of a little square-angled, rock walled bay, on the south point of which it rises as much as a hundred feet above the lake. On both sides of the cutting mass the black gabbro is filled with large angular masses of the same coarse anorthite rock. The included masses sometimes reach many tons in weight, and in some places predominate over the including gabbro, which then appears as if veining the coarser rock. At the west angle of the bay the included masses are nearly absent and the gabbro resumes its usual vertically columnar appear^ ance. At the north angle of the bay the anorthite rock rises again to a hight of over 150 feet. The inclusions of angular masses of the anorthite rock in the gabbro indicate the more recent origin of the latter, and this conclusion is borne out by the section made from a specimen taken at. the contact with ♦Copper bearlnti Uocks MonojJiraph V U. S. G. 8, page 302, S a 'A tt' -15 ^ I -■,"'' CA R LTON PEAK /" ■_'-,"/ .»^7/y, /•//)())■»' fAc I /the - y^;! ■Hi. V'T Irar inJetenninat l\J i'tj ' Trrrace soft. jLakf Suner-ior K r * ■^ " '■ \ i 1 » .61 > ^ ' -V 11 - ■- * •x '' ' - ' 7-'^ > " / ^ ^ /."__■, 'A .^ - > - ' .-' ■-■ 1 ~ — "^ 1 Y. _'■ ' - ^ '" ''^' — ; t " " / •/. ;/ - '^' 7. , - ^ /j- ^ •.' - X"-- \ ■►^ fS-3 ^.. '",• . ~g'* I ' a J5, .1 ' , :^ C"^ ■ il^ V. ' "' / ■; 1 ■« •.:v l^ake J> iipertor V. f- I*" X THE ANORTHOSYTES OF MINNESOTA. 13 the gabbro. in which relatively fine gai)bro surrounds the ends of the anorthite crystals as the Ixiso o' any porphyry does the porphyritic crystals which lie imboddea in it." This locality is a dilTerent one from that described by the writer as the cove immediately below Split-i'ock point, but the general conditions are the same. Notwithstanding the evid- ence of the included masses, Irving seems to have clearly re- garded the anorthosyte as eruptive thnnigh the gabbro. which is probably the same geologically with the rock which the writer has recognized as a diabase porphyryte. For he saya in another place* "This anorthite-rock presents very interest- ing occurences, as described in a subsequent chapter. It ap- pears both as masses cutting black gabbro, and as included angular masses in the same rock'.' This conclusion as to the eruptive character of the anorthosyte appears to the writer to be an error due to hasty or imperfect observation. Irving no- where cites evidence of its eruptive character. V)eyond that it appears suddenly in the midst of his Koweenian tlows, a state- ment which is quite true, but which is, of course, susceptible of a totally different explanation. There is positive, observ- able evidence of the Keweenian tlows resting on the surface of the anorthosyte and being traversed by dykes of the same ma- terial as the Hows. This fact taken together with the ab\md ant inclusions of the anorthosyte fragments, which arc not always angular blocks, but frequently rounded boulders, leaves no doubt whatever as to the relative age of the formations. WinchelVs vieios:—Pvot N. H. Winchell's views as to the oc- curences at Split- rock may be gathered from the following ob- servations taken from his notes published in the ninth and tenth annual reports: -'The west side of Split- rock river at its entrance to the lake is low, but the east side, or north-east- erly, is high, and formed of a basaltic bluff or rock which ap- pears on the immediate coast at a short distance east of the river. It there embraces a large block of a whitish-looking rock, which at a distance appears to be a granite, but which in reality is what has been described by Norwood as feldspar pro trading through greenstone. This does stand up like a dyke, but is in reality older than the trap, and occurs generally fur- ther inland, forming hills several hundred feet high. This bluff rises sheer from the water 13() feet and has basaltic dark trap on each side of it, the rock itself being massive. On the *Op. cit. p. .59. ' 14 HULLETIN NO, Vlll. east sid(3 of this high rock the trap shows inchided masses of the same rock, a fact which Norwood mentions, but yet speaks of the feldspar as a protuded mass of later date than the trap."* AiiorthoHijfe (Jon founded loith thv Kcweenian Eruptires: — It will thus be seen that Winchell correctly interpreted the pheno- mena as indicating that the anorlhosyte is an older formation than the eruptive with which it is associated. But he still regarded the anorthosyte as a member of the Cux)riferous or Keweenian series. For, in the report for the following year he gives the following formulation of one of his conclusions: "The feldspar masses are of the same rock (geologically) as the Rice Point gabbro (V), and both are the result of copious and perhaps one of the earliest, igneous outflows of the Cupri- ferous. The more copious the igneous outllow, the coarser the resulting crystalli^iation and the higher the hills formed, as well as the purer the labradorite material. The later out- flows derived fragments from the "clinker fields" and from the knobs of feldspar already formed, as they passed along f Thus none of the earlier observers, Norwood, Winchell or Irving, has differentiated the anorthosyte masses exposed at Split- rock from the general aggregation of volcanic flows which constitute the Keweenian series on the Minnesota coast. Occurrences at Beaver Buy: — At the next high point on the shore to the east of Split-rock point the same conditions, as far as regards the inclusion of masses of anorthosyte in the dark lavas is repeated; but it is not till Beaver Bay is reached that the anorthosyte is again exposed in place. Here, it may again be clearly observed in clean, wave-washed cliff sections to be the basement upon which the lavas were extravasated, and to have here also afforded numerous boulders and blocks which were caught up and enclosed in the lavas. All former notes as to the occurrence of anorthosyte at Beaver Bay are singularly de- ficient in geological information. Irving enters in some detail into an account of the geology of Beaver Bay, and publishes a special sketch map to show the distribution of the various kinds of rock, but he barely alludes to the anoi'thosyte.j Wincheirs most important note is as follows: "Between Beaver Bay and the Great Palisades are numerous feldspar masses, in the coast series, and inland from the shore, a very short distance, is a range of low hills made up of feldspar with trap rock on the flanks. "§ ♦Ninth Annual Ueport Geol. and Nat. Hist. +Tenth Annual Report Geol, and Nat. Hist. JCopper-bea-'lnK Rocks, pp. 30(1, 307. tSNtnth Annual Report Geol. and Nat. Survey Minn.. 1880, p. 30. Survey Minn., 1881, p. 114. Hist. Survey of Minn., 1880 p. ;i4. c I" • '4 2. - P ^ ^ 1 M J ( — Pf^ 1 B-Jr ^ll ^n^HHPH X- .,».^ 1 ■ '■ . VlJj 1' mf ™ •5 **-"■•• ■ . '"'. i i 1 HI . JJl f ""'* , •z i Kf :. ilil .1 1^.;.,^ ., \ f ^UH^i 1 , >. / Wjtrf^^ ,■ I . ^^^R^ ,' .'z ii,f^^5^lL. \m§% ■in m^ ;., 7 7 7 .^If^'iT iJj ■' ':' >« ^ <■ I'l m ■ <^. ^"«Sp% ■t. t^: ', J r ■:■ ■ ^^^jMrWli j^ HH^BIi il'l ] ^' '«> - a'i .•> -^ . H li "f ''> ^ 7t* J b 'li^HH^^I^HPAi' i<: ■ {• '" -' i^iaS .: 5 U ■■^p^fmmL'i^sMr.^^^^m^^mmiir :. - " '4 vn">B 7 r Nr ■ 4k ^m^^imm , '/^ySj^K^^I^^^^^I^^HBf '^* >. :: ''^''^^^K^flH^^^^H^ _5 ' ^ ''^3B$Ji^B^B^^^^^t ■ n * 1' . Jdj^HKL^^^^^^^^^^^^^Hv Z r: jgjfeTtS^M'aBJ^^^^^Bl 5^ — W' Xm^^^^^^EI -r*^ • '^^^Hl f •r— ' — ^^^vH r T *-• ^IrjMJV — j^ y. ^H^H £■" - ^^K o x_^ y ^V' • £ tl. ; ^m ■ .. , ,u4J 1^ " ■^ . ..-j^mm. ." ' '•./ J < ^ THK ANOUTm)SYTKS UF MINNESOTA. 15 I ^m -'' At Beaver Ray there ar(> four separate and distinct occurrence* of the anorthosyte. The first of tliese is at tlie shiiijjrlt' ''nvo at tho extremity of the soutli west lieudland of IJeaver buy. and on the south sid(« of the vortical cliff of nid porjihyry which hero forms so striking a landnnirk. The anorthosyte (xtcupies at this place an an»a of about (KiO l)y iJOO feet. 'J^he greater part of the mass, which rises to about 30 feet above the level of the laU(!. is the normal coarse wliitish anorthosyte free from any special structural features. On the side facing the shingle cove, however, i. e.. at the north-eaHtern extremity of tho mass, a pronounced banded structure is apparent, resemb- ling som(^what that d(?seribed in the vicinity of Si)litrock point. The stratiform apftearanceof themass is well illustra- ted in Plate III. The di[) is to the southeast. Tlu; stratiform structure is here duo to the fact that there are certain sheet-like layei's somewhat richer in pyroxene than the rest of the rock. Tho decomposition of tho pyroxene has tho effect of staining tho hiyer so atlected a yellowish or greenish rusty color, so that it presents a strong contrast with tho unal'focted portions of anorthosyte which he between the layers. The decomposi- tion of the pyroxene has not chemically affected the consti- tuent feldspar of the rock oxc(»pt to a very limited extent, but it has the etToct of mechanically disintegrating it so that it. crumbles readily in the hand. Hence, as may bo soon in the plate, the dark layers weatlior out as grooves (n- depressions. Under the microscope the feldspar in specimens from tho dark bands is fresh, save for a few points whore decomposition products appear. It is. however, much cracked and the yellowish decomposition products of the augite are distributed along tho cracks. Norwithstanding thf> presence of those cracks these feldspars show no strain phenomena such as un- dulatory extinction, faulting, shearing, cataclastic structure, granulation, etc. The original allotrioraorphic granular structure has 'not boon disturbed, and it is highly improbable that banding is in any way associated with shearing action after the final solidification of the rock. It seems to tho writer to be essentially due to some local chemical differentia- tion, associated with movement, in tho thickly viscous magma prior to crystallization. This stratiform facies of tho anor- thosyte is at this place cut by a well defined dyke of olivine- diabase, which forms the jointed rock on the right in the illustration. Tho dyke is about 25 feet wide, is nearly vertical and has a strike corresponding with the general trend of the 16 BULLETIN NO. VIIL coast. The anorthosyto is observable on both sides of the dyke. The latter has weathered out more easily than the anorthosyte, and its place is therefore marked by a depression, or negative dylie profile. In following this depression south- ward the olivinediabase of the dyke is covered with great, loose blocks of the anorthosyte which have fallen from the wall on either side. To the southward the anorthosyte gives way to a mass of hypersthene-diabase in which are enclosed many huge blocks of the former rock. Some of the.se V)loeks are 20 feet in diameter. The second occurrence ol the anorthosyte at Beaver bay is on the north east side of the south headland of the bay whe:^ it forms two islets, as noted by Irving' The ihird occurrence is on the nrjrth west shore of the bay, extending from the base of the sand spit for over three eighths of a mile north- eastward. The shore contour is serrated with alternating little rocky coves and points. In the bottom of nearly all the coves, as well as on several of the points, the anorthosyte maybe seen in rounded rochcs )twutonn>'('s surfaces, with the diabase which predominates on the points either rest- ing on it as a mantle, or eruptive through it in the form of aykes. The anorthosyte is practically continuous from the base of the sand spit for the distance above mentioned. The gracefully rounded forms of the old surface of the anorthosyte where it passes underneath the Keweenian flows is especially interesting and is well illustrated in Plate IV. Another contact showing the same relationship is illustrated in Plate V. The humraocky and roches moutonnres aispect of the old anorthosyte surface is entirely analogous to the phe- nomena whicli the writer has elsewhere* described as prevalent at the contact of the Keweenian (Nipigion) and the Archaean in other parts of the lake Superior region. The anorthosyte is traversed here not only by dykes of dia- base but also by the common, red, acid eruptive of the country (quartz porphyry and granophyre) which, in the form of irreg- ular dykes, intersects the older dykes of diabase. The diabase which lies on the anorthosyte holds many lartre blocks and boulders of the latter of the same character as those i:)re- viously described. It is strongly amydaloidal in places. ♦Note on the pro-palii-ozoic surface of tlie Archu'iin Terranes of Canada, Bull. Oeol. Soe. Am. Vol. 1, pp. l(ia-174. ' >Jl:!.i,.; i' J ' T.' ■«■•,.■•"■■)•; «■ .' •■ ^ » " \ - ' '' '■ \, '. I y y y fs ir tl [8 tl tl P g w w tl ■w h o si si y o o h C a r; s^ d is s P s h d q t^ I r V t t THE ANORTHOSYTES OF MINNESOTA. 17 The fourth occurrence of tho anorthosyte rock is at the first falls of Beaver river and is described by Irving in the follow- ing paragraph: "At several points along Beaver river this black gabbro carries large masses of anorthiterock I anorthosyte] similar to that described as occurring further up the coast. The boulder like character of the anorthite-rock, though often pronounced is not always so plain and in some places it looks more like a dependency of the prevailing black gabbro.* A careful inspection of the occurrence by the writer enables him to state that there is no reasonable doubt whatever, of the foreign and included character of the anor- thorsyte boulders Shore below Bearer liaij : — The next locality down the coast where the anorthosyte appears is at a point about a mile and a half below the north east headland of Beaver Bay and directly opposite a small island which lies about a quarter of a mile off shore. Here the anorthosyte emerges from beneath the sheeted trap rocks, and has an exposure of over one hundred yards in the form of bold, yellowish- white domes. It also occupies the island above mentioned which is about one- eighth of a mile long. The island is traversed by two vertical dykes having a strike transverse to the general trend of the shore. One of these dykes is 200 feet wide and is of the character of a diabase or gabbro. very dark in color, coarse in texture and rich in iron ore. The second dyke is quite small and may be simply an apophysis of the larger one. The contact of the dykes with the anorthosyte which occupies the mass of the island is sharply observable as a vertical plane with the intru- sive rock somewhat finer grained near the dyke wall than at points near the middle. A quarter of a mile further down the shore the anorthosyte again appears in an exposure several hundred yards in extent and is here also clearly intersected by dykes of the dark diabase. Half a mile further on at a point on the shore about three- quarters of a mile beyond the east end of the island above men- tioned, the anorthosyte is again exposed; and here, just as at Beaver Bay, the domed surface of the old pre-Keweeniau ter- rane is seen to be capped with a sheet of amygdaloidal diabase which adjusts itself to the curvature like a mantle. Irving's observations on these exposures arc summarized in the following note: "At several points in this vicinity the black rock was observed to include masses of coarse anorthite- ♦Oopper-bearinK Rocks, p. 306. S 18 HULLETIN NO. VIII. rock. Tho latter did not appear to occur here' in boulder-like masses, but rather in irregular outlined areas. At one point on the shore of Sec. 6, directly west of the island above referred to, the white, anorthite-rock rises like, a dome in the black gabbyo, which is seen above it and on both sides of it* The southern point of the island is formed of anorthite-rock; and due north from this point, on the mainland, is another area of white rock ap- parently trending north and south." It is difficult to under- stand why Irving failed to api)reciate the fact that the anorth- osyte areas represent the eroded surface of a pre- Keweenian terraue. Baptism River: The next locality where this interesting for- mation appears is on the Bajitism river, about half a mile up stream from its mouth. In the vicinity of the foot-bridge, which here spans the rocky gorge of the stream, the anortho- syte is observable both in large masses not fully exposed, and in the 'form of boulders several feet in diameter imbedded in vesicular and amygdaloidal lava. On the Slope of Saw-teeth: — Farther along the coast about two miles and a half below the mouth of Baptism river, the anor- thosyte again crops out on the summit of a forest-clad hill about half a mile inland from the lake, and 800 feet above the level of its surface. The exposure has an extent of 100 by 25 feet and its relations to the flanking eruptives are not revealed. Carlton Peak: — The next occurrence of the anorthosyte is at Carlton peak, a little below the Temperance river. This ex- posure is the most extensive and jjrobably the most important of any on the coast. This fact that the summit of the peak was composed of this rock was tirst announced by Norwoodf who stated that it was "composed entirely of feldspar rock" but imagined that it was a huge dyke. Irving makes the following statement regarding the occurrence: "The high bluff known as Carlton's peak, near Temperance river, shows at its summit numerous large angular fragments of anorthite-rock, such as has already been described in connection with the Beaver Bay group. None was seen that could certainly be regarded as in place: nevertheless the mountain is without much doubt, composed of this rock, and I should regard the rock as having antedat- ed the Temperance group flows rather than as a cutting mass." It is quite apparent from this statement that Irving himself did not visit the summit of Carlton peak, and must have based his *Tho itullcs are the writer'^s. +Hcport of the Ueol. Survey of Wisconsin, lovvu imd Miiiricsota by I). 1). Owen p. 380. THE ANOUTHUSVTES OK MINN LOSOTA. 1!) statement, as to the occurrence there, upon the imperfect ob- servations of others. The writer ascended the peak, and not only confirmed Norwood's statement that the summit is com- posed of anorthosyte. but also found that the whole upper half of the mountain is made up of the same material. The expos- ure is an exceedingly bold and strikinj; one. The peak is 027 feet hi^^h above the lake and al)Out a mile and a half distant from the shore. It rises in the form of a ,u'r<'at dome and is a very conspicuous land-mark: the domed aspect bein.ir, however, confined to the upper part of the mountain. The mountain is flanked on all sides by the sheeted lavas of the Keweenian and these, by their gentle dip lake ward, ctmstitute an easy grade to the declivity of the dome, nearly half way to the summit. This gentle slope is heavily timbered and in part terraced, but has been burnt in places; while the upper half of the mountain is practically a continuous bare surface of anorthosyte, pre- senting to a i-emarkoble degree of perfection the roches mou- toum'cs curves down to where it plunges beneath the mantle of lavas which Hank the base. The mass has none of the charac- ters of a dyke or intrusive boss as Norwood supposed. The relations which obtain are illustrated in the accompanying dia- grammatic section Pig. -.* The area of anorthosyte exposed at Carlton peak is probably more than half a mile square. Beyond Carlton peak no other exposures of anorthosyte have been observed down the coast, and none are known in Canad- ian territory in the region around the lake. GEOLOGICAL RELATIONS OF THIC ANORTHOSYTE. Pre-Keweenian Age: — In the foregoing pages sufficient evi- dence has been set forth to demonstrate the pre-Keweenian age of the anortliosyte. This evidence is four fold. (1) The anorthosyte is traversed by dykes of the Keweenian eruptives both acid and basic. (2 ) The Keweenian lavas hold imbedded in them innumerable boulders and blocks of anorthosyte evi- dently detached from a pre-existing terrane. (3) The anor- thosyte forms the surface upon which the Keweenian lavas now rest, and upon which they were originally extravasated. (4) The anorthosyte affords both by its petrograidiical character and by the nature of its surface the most satisfactory evidence of profound erosion prior to the extravasation of the Keweenian eruptives. I)0)iH'd and ITiiiivnockn C/ia racter of tin' Pre- Keiveenidn Surface:— Not only does the surface of the anorthosyte demonstrate the erosion of that formation in pre-Keweenian time, but it shows *Puclnsj patte l;J. 20 UULLKTIN NO. VIII. that the Keweenian lavas have only to be stripped from its surlace, by the ordinary forces of denudation, as has been done in several instances cited, in order to att'ord us the typical domed, humniocky, and ror/ics niouto^uieefi surface which is usu- ally ascribed to glacial action. This type of surface prevails over the greater part of the Archajan terranes of Canada and passes in this peculiar condition under the base of the palajo- zoic column wherever the contact has been observed. The writer has in a former paper inferred from this fact that the hummocky character of the Archjcan terranes of Canada is not, as is commonly supposed, ascribable wholly to glacial action in Pleistocene time, but that it is essentially pre-pahoozoic in its origin, being only modified by Pleistocene erosion.* The beautifully clear evidence here recorded and illustrated (Plate IV) as to the domed and roches nioutonnees surface of the pre- Keweonian rocks of the Minnesota coast where they pass under the Keweenian lavas, is in harmony with the descriptions and conclusions given in the paper referred to; and demonstrates that in this regard the anorthosytes have one pronounced fea- ture in common with the Archasan of other parts of the lake Superior region. Interval of erosion:— The character of the anorthosyte as set forth in the petrographical notes is such as to warrant the as- sumption that it is a plutonic formation, and that it solidified in the form in which we now see it under deep seated condi- tions; and that therefore the amount of erosion necessary to reveal it at the surface must have been great. The interval in which the work of erosion was effected, was probably the same pre- palaeozoic interval as that which effected the reduction of the Archaean to the great hummocky plane which occupies so large a part of the North American continent, and which re- veals at the surface the great areas of Archajan granites. Absence of the Animikie: — The recognition of a pre- Keweenian terrane on the Minnesota coast involves certain consequences of geological importance. One of these is that the formations of the Animikie so largely developed in the vicinity of Thunder bay, and there underlying the Keweenian, are here wanting. Former writers have assumed the existence, beneath the Ke- weenian, of the Animikie rocks along the entire coast. But the repeated outcrops of the anorthosyte at the base of the Ke- weenian shows that for the middle third of the Minnesota coast, at least, the Animikie, is wanting. The relations thus become *Pre-Pala>()zoio Surface of the Arclia-au Tunancs of Canada. Bull. Geol. Soc. Am., Vol. 1, pp. 163-174. THE ANOHTHOSYTEfS OK MINNKSOTA. 21 iUO- entirely analogous to those whu-h obtain on the Canadian por- tion of tht! lake Superior coast beyond Thunder hay where the Animikie is wanting and the Keweenian (Nipigon) rests di rectly upon the humniocky surface of the Archfean. Shallowness of the Keweenian: — Another important conse- quence, resulting from a recognition of the true character of the anorthosytes. is a correction of Irving's estimate of the thickness of the Keweenian. In his account of the stratigraphy of the Minnesota coast. Irving places the thickness of the Keweenian series at i\),000 feet, stating that, in all probability, it may reach 22,000 or 24.000 feet.* Disregarding the first group, the St. Louis gabbros, for the thickness of which he admits he can give no good figures, he places the thickness of the remainder of the Keweenian at 17,000 feet At Split-rock he places the thickness at 10. 000 feet, and at Temperance river the maximum of 17.000 is reacthed. There is something sadly astray with these estimates, and with the stratigraphy upon which they are based. According to Irving the Keweenian series on the Minnesota coast is thickest along its middle third. It is along tliis part of the coast that the underlying basement, upon which the Keweenian rests, crops out sufficiently abund- antly to demonstrate that the series is comparatively thin, ranging from zero, locally, up to a few hundred feet. The Keweenian on this coast is by no means the excessively thick series that it has been represented to be. In tlie opinion of the writer, its maximum thickness is not more than one- tenth of the value at which Irving placed it. All estimates based upon the dip of the lava sheets are fallacious, unless it is clearly recognized that a large jiart of the dip is the original slope of the surface over which the lavas llowed; and that there is a constant and scarcely avoidable danger of piling up contemporaneous flows, one on top of another, by an undue extension of imaginary stratigraphic planes. In this way figures are obtained foi- the thickness as enormous as they are absurd. The stratigraphy of volcanic flows and instrusive sheets, traversed by many dykes, is not so simple a problem as Irving seems to have regarded it. and his analysis of that stratigraphy, his subdivision of the Keweenian into groups, and his estimates of the thickness of the various portions of the series are of little value; a statement which it is as painful to make as it is necessary in the interests of sound geology. *C'opi)or-l)oiiiiiig HoL'ks. p. 26C. 22 liULLKTlN NO. VIII. Correlation mid nnmr nf f/ic foniiafioii: — Petroj^'raphically, tlie anoi'thosytos of the Minnesota <;oast. aro clo.sely alliod to cer- tain gi'(!at uiass(»s or areas of pla«^ioclase rocks which prevail in the Province of Quebec, and which are known al.so in the Adirondack's and in New Jersey. Adams, who has given many years of study to tlie (Quebec occurrences, has shown that tiiey are irrupt ive masses breaking- throufi^h the ordinary <,'n(>isses of the Archioan.* It woukl app<>ar, however, tliat they Ions; antedate the ad\'ent of the pahoozoic, that they were phitonic intrusives in the Archioan prior to the great interval of erosion which separated the Archioan and Pahcozoic, and that their appearance at the present surface of that region is duo to pre- pakooz familiar with lalcc Suporior f?an. wiio in his description of the rocks of the north shore of lake Suporior *rofers tlius to the formations now known as the Animikie s to the plane of the stratiti(!ation. and the crowning overflow gives a jjeculiar aspect to the whole region occupied by the formation to which it belongs.'' Practically the same language was again used by Logan in his account of the g<'ology of ('anada published in 180;J. Bell ado|)t(Hl the same view, that these trap sheets are sur- face volcanic flows, but recognized that the.v are. in part at least, later in age than the bulk of the Nipigon rocks, from the fact that the sheets in jilaces cap the Nipigon strataf. Irving objects to the idea of any single ••crowning overflow," but clearly regarded the greater i)art of the trap sheets as contemporaneous volcanic sheets, as may be gather(*d from the following (juotaiions: "So far. then, as 1 have been able to learn from original observations, and by reading in the light of the observations the accounts of others, the Animikie rocks of the Pigeon river- Thunder bay region consist of a great series, probably upwards of 10.000 feet in thickness, of quartzites. which are often arenaceous, (puirtz slates, argillaceous or clay slates, magnetitic quartzites and sand-stones, thin limestone beds, and beds of a cherty and jaspei-y material. With these are associ- ated in great volume, and in both imbedded and intersecting masses, several types of coarse gabbro and tine grained diabase, all of the types being well known in the Keweenian series.! Again referring to Bell's stratigraphical scheme for the Animikie. Irving says: 'Then agam. the great ♦Report of I'rosri'S^^, drol. Survt^.y of Ciiiisulu 184(>-? p. i:i. tGeol. Survey of Oaiiiulii Heporl of I'lOtrress IWiB-tiO. Wopper-beiirltig Rdcks, i)..TOt. 26 BUI.LKTJN NO. ^'IIT. I f ■( volume of: included beds of gabbro and diabas*' is almost entirely ignored. In the third division of his ticheme it is said that trap beds are associated witli tliese rocks along the north shore of Thunder bay, at the Thunder bay mine, and in the township of Mclniyre, and yet the whole volume of this divis- ion is placed at only 4r)0 feet. But as seen, all the wny from Wau&waugoning bay, on the Minnesota coast, to the south side of the Kaminisfciquia valley, and again in the E'igeon river country of Minnesota, these included l^eds must aggregate over a thousand I'eet (?) *while they may be nuch more than this (V ?).f This importavit omission is pi'obably to be ex- })lained by Bell's having regarded all of these beds as part of the so-called "crowning overflow" which is supposed to have taken place after the accumulation and i-emoval of thousands of leet of the newer Keweenian or copper-bearing strata. "The only evidence of any such general overllow consists in the similarity of the crystalline rocks found capping the hills in diiferent parts of the region. Not only is it much more in accordance with the geology of the lake Superior region to suppose these occurrences to represent many different flows, but there is distinct evidence that they do so in many cases. This evidence consists in ])art in actual visible inters tratiflca tion with the slates in some places of great beds o'" olivinitic gabbro identical both macroscopically and miscroscopically with the rock capping Tliunder cape. i\.noUier evide.ice is the very great irregularity of level of this .supposed How must occupy, vhe liight at which it is found varyin.'.r back and forth through distances of several hundred feet. Yet stronger evidence is found in the general structural character of the region, by virtue of which each lieavy, enduring, ciystalline rock layer constitutes a ridge with a long front slope and a preclptious back slope "j. Selwyn, in 18H4, adopted the same prevalent views as to *he partial volcanic composition of the Animikie group. Writing in that year, he says: "Between Thunder bay and the east end of Nipigon the three series (Animikie, Nipigon and Ke- weenian) follow each other without ap] parent unconformity dud dip at generally low angles towards t'le lake. Up to the summit of the Nipigon series there are many larger inter- stradfied beds of columnar diabase, then follows the Keweenian 'The ((iicries ari> ilii' Miitei's. *T!ic (nierU's lire tlie writer's. ♦Op. CIt. pp ;)HI 382. AI'COLITIC SlI.Lh. 27 series, consisting', ote * * * The absence of paheoaLoiugicai evidence of a^re may be, )ierhai).s. in a ,<,'i'eat measure, accounted foi- by the great and repeated manifestation of volcanic activity over the whole rejjfion durin<^ the accumulation of the sedi- ments, producing conditions higjily unfavorable foi- the exist- ence of animal life."'- From various references scattered throu.i^h th(! vvritin<,'s of Murray, McFarlane. Hunt and Mc- Kellar it is clear tViat all of these observers regarded the sheets associated with the xNniraikie slates, sandstone, etc.. as con temporaneous volcanic flows. The late Prof. Alex. Winchell in speaking of tlie trap sheets which cap the Auimikie slates in the vicinity of the interna- tional boundary, used th(? following v/ords: "The great gab- bro Hood. 1 agree fully with American geologists in assign- ing a primitive molten condition to the sheet of gabbro which covers so many hundreds of square miles it; the northwest. Bat its wide extent considered as a molten flood, is a f al!i, Hull. No. V;.(}ool. aiul Niit. Hist. Survey of Miiiiio.sotu. l«n,p. 114, IIGeol. ."Purvey of (JaniitlH, tifnimil iv.jon 18SH, part )I. SOp I'lt. p,S 28 BULLETIN NO. VI I L "the rocks comprising this silver bearing formation (Aniiuikie) consist of basic traps, black and gray argillites, cherts and jas- pers, with some ferruginous dolomyt,es etc."* The evidence of the intrusive character of some of the :-heets, which Mr. Ingall adduces, will be reverted to in the sequel. Ic thus api)ears from a review of these references to the writings of most of the geologists who have had occasion to to become personally familiar with the Animikie group that the uniform teaching has been that the series is partially com- posed of volcanic rocks. Most observers have r<'i.''i' one which the writer is disposed to deny. The writer believes that he has a field knowledge of the Animikie rocks not less extensive .than that of any other geologist who has heretofore written upon then. He Icnows them fron^ Guntlint lake to the mott easterly islands of Nipigon bay. He has examined their entire exposure, in all its magnificence, along the coast of lake Superior from Grand Portage bay to its most easterly occur- rence on the main shore. Traveling on foot, he has examined all the sections which are revealed on the line of the Canadian Pacific railway, both east and west of Port Arthur. He has followed the line of contact of the Animikie against the Archaean probably more closely than any other geologist who has put his observations on record. He has visited and descended most of the important silver mines that have been in operation during the last six years. He has examined the well knowji exposures on the wagon and canoe route from Port Arthur westward by wa\^ of Rabbit mountain, the Palisades, Silvei Mountain, Whitefish and Arrow lakes, and the lakes of the international boundary. He has also traversed the Kam- iniptiquia river in canoe, and has made many minor excursions throughout the district occupied by the Animikie rocks.t ♦ 1(1 p. 23. ■''riic VMi'louH evaiuliiations wiiit'li tlit' wriler liiis timrte fioin I iini' to liino of tli<' Atii- iiilUic loi'ks liii ve 1)1 L'ti iiiciili'nta'. to, oi' iipait from, his niii.ioi' work Jii Uk; In Wo Super ior rcjiion: and liis kiiowlt'dfii- of tlio Hcrii's lias, tlit-rt'fore hceii Kalhfrcd pi»'ci;-iiu>al. and ha- li'd to no sysiciuatlc ai'i'otinl of iis sjcolojrlt^'al fi'alurcs. Tlic last iioi^ossary and iniporlani inforniaiiori was olilalni'd whUc ixatnlnini? the I'oast of the laluiirl/.-k(^niliipli> ivof PiL't'cm point, by \V. S. Bay- k\V Am..I()ur. of Sci.. Vol. XXMX April. US'.)(). till ciiie insrsiuoe dht' w riu-r lias observod an aiuyjidiiloidul rock in the Aninilkie prov- inoo, but this Inis no connection with tlie trap sht-ets and is probably an out-lylnjc I'eninant of tin' Ki^wei-nlan. I'f. Note oii tin- oiuMirreuco of native copper in the AuitniUi.- locks nf Tlmnder hay. The American Geoloffist. March, ISOl). 82 BULLETIN NO. S'lll. columnar structure these trap sheets are entirely analogous to the numerous dj'kes of the region, in which the columnar structure is pronounced from wall to wall of the dyke. Petrographical differentiation —While thus emphasizing the con- stancy of the petrographical character of these sheets, it is not the intention to assert that there is no yjetrographical differen- tiation within the sheet itself. Theie is. indeed, a very pro- nounced and evident variation in the character of the rock comxjosing each sheet. But this variatioi; obeys a definite law. and Is constant for every sheet, so that the variation itself be- comes an invariable character, wherever the full thickness of the sheets may be observed. This petrographical variation is strictly analogous in its physical aspects to the dilTerentiation which the writer has described as characteristic of the dykes which traverse the Archaean terranes in the Rainy lake region.* Whether there is also a corresponding chemical variation as in the case of the dykes, has not yet been investigated. Where- ever the contact of the trap sheets with the Animikie slates has been observed, whether at the lower or the upper surface of the sheets, the diabase at the immediate contact has been found to be a dense compact rock, either microcrystalline or quite aphanitic with occasional minute phenocrysts of plagio- clase scattered through it. The texture of the rock becomes coarser rapidly as the distance from the contact is increased, and if the sheet is thick the rock is usually quite coarse at the distance of a few yards from the contact. In the structure, as exhibited in a number of representative slides from different localities, the rock varies from that of a dense, very fine grained diabase-porhyryte into that of a fine grained diabase, the ophitic structure being pronounced, and then to that of a coarse diabase with a tendency in places to assume a granular structure. Analogous, though somewhat different, variations in texture and structure, also obtain for the sheets of red granophyric quartz porphyry, at least so far as their lower contact is con- cerned, as may be clearly observed on Victoria island. These variations in texture and structure, particularly in the diabase, are well known to be characteristic of the contact phenomena of intrusive masses; and taken together wit^ the strong simi- larity in the field aspect of the sheets tc the dykes of the region, the presuL^ption in favor of regarding them as hori- zontal dykes or intrusive sills becomes very strong. ♦r'lilrographical niffeiiMitiiiiioii of Cortaiii Dykos of the Kainy Lake Region. Amcrivan Geologist, March, 1891. Tht; LACCOLITJC SILLS. ;3n SOME MH(JAI) FEATURES OF THE TRAP SHEETS. Their Slnii'lcUi/- -Another argument in favor of the intrusive character of the sheets is their simple and uniform character regarded as geological masses. In this they again resemble the great dykes that traverse the region. In the case of sur- face flows, whether from volcanic craters or from fissure out- wellings, it is an exceedingly abnormal condition of alfuirs to find the extravasation take the form of a sinr/lc sheet, occupy- ing hundreds of sfiuare miles and of practically uniform thick ness. Yet tliis is precisely the character of the sheets under consideration. Their simple individuality, their regularity, the total absence of over lappings of one sheet oh another, are features which are sufficient to negative the supposition that they are surface tlows. The unity and persistence of the sheets over wide areas is remarkable. The Animikie strata and these associated trap sheets have together been dislocated by a great system of faults; and the orographic blocks so formed have been vei'y frequently tilted, so as to present a long gentle slope to the south-east, and a steej:) scai'p to the north- west The angle of tilt has been small ranging usually from 0° to 5°, but is sometimes higher; and portions of the region afford a remarkable illustration of this tilted structure, and of a topography conditioned by it. The many long, narrow lakes which occupy the Animikie province in the vicinity of the international boundary clearly lie in fault lines, between a steep scarp on one side, and a more or less gentle slope on the other. The non-recognition of this prevalent tilted structure has been the origin of much confusion in the descriptions of the Ani- mikie group; and very excessive estimates of the thickness of the series have been made both by Irving* and by Ingall.t In the opinion of the writer one fifth of these estimates is much nearer the true thickness than the figures given by these geologists. The recognition of the tilted structure renders the correlation of discrete portions of a single sheet a matter of no great difficulty to a stratigraphic geologist. Such a corre- lation, based upon correct ideas of the structure, shows cleaiiy that the trap sheets are few in number, and single sheets may be traced in geological continuity for many hundreds of square miles. This persistence of single sheets, more noticeable in the thick "trap caps" than in the thinner intercalated sheets, is even ♦lO.O'O fw!t. Coppt'i'-heiirin? Uocks p. HSO. tl2,iKH) feet, liepcirt on Mines and Minius la Lake Superior p. ^6. 84 HULLKTIN NO. VIII. more evident on the lake front, whore tilting is le.ss pronounced and the sheets may be traced in actual, as well as geological continuity, over equally great areas with practically uniform thickness. In other cases, where the level-topi)ed mesas are dissected by erosion and separated into several topographical masses by valleys of greater or loss width, there can be no reasonable doubt of the original continuity of the separate caps of the now isolated hills, since they occupy the same levels, overlie the same rocks, have the same general thickness, and the same peculiarities of petrographical detail both macrosco- pic and microscopic. This persistence and the practical uniformity of level (prior to tilting), together with the fairly uniform thickness of the sheets constitute a combination of features not affected by surface flows. Absence of Pijrocla-stic Rocks — The sheets are nowhere asso- ciated with pyroclastic rocks such as would suggest their ex- travasion at the surface. There is no ash, or tulf, or coai'se agglomerate. Neither is there any trace of breccia or rece- mented lava fragments of any kind, such as are commonly de veloped in the progress of a surface flow of lava. Absence ofjiow slrucfiire — There is, moreover, a complete ab- sence of flow structure such as is so abundant in the lavas of the Keweeniar This is true of the surface of the sheets as well as of their internal parts. The surface of the Keweenian lavas is very frequently characterized by the presence of wrink- les and of other manifestations of what is known as "ropy structure." These features are never found on the surfaces of the trap sheets, even where these are freshly exposed by the recent removal of the over- lying slates. The enclosing rocks — Another feature which does not accord with the supposed surface character of these sheets is, that the sedimentary strata below the sheet are usually, so far as ob- sei'vation is possible, the same as the strata which immediately overlie it. Of course, as regards the thick trap caps of the region, there are generally not now any strata reposing upon them, and it is, therefore, impossible to say what the nature of the overlying beds originally was. But as regards the inter- calated sheets, the statement is true that there is no essential difference in the character of the enclosing rocks above and below the sheet; and on the surface of the thick trap sheet, which extends inland from the town of Port Arthur and skirts the north shore of Thunder bay, and which is, for the most part a cap sheet, there are numerous remnants of the overlying f.ACCOLITIC SILLS. 35 Aniraikie slates still adhorini,' to the trap (soo Plate VT. Fronfifi- pivcr) Vv'hich aro the same rode essentially as t. e slates which uiulerlie the sheet. Intersertion of stnihi Inj thr s//rr^v— Ovvinj? to the little disturbed couditioii of the Animilcie sti-ata. and to the fact that the sheets had their origin prior to the disturbance, the sheets appear (•(jmmonly to follow in each ease a sinortant ])assuge from one geological horizon to a very ditlerent one. Many of these intersections of the Animikie strata by these trai> sheets have been observed and recorded by Ingall.* One or two cases have been noted by Irving, t and others have been observed by the writer. One of these intersections is well illustrated by Ingall in the lower figure of the i)late facing page 24 in the report above referred to. Although this is entitled an illustration of a "Trap flow on Argillytes;*' it is. undoubtedly, an intrusive contact, as Mr. Ingall himself is dis- posed to admit. In addition to the observations of Ingall and Irving a few other cases noted by the writer on the coast may here be given.' On the face of the high bluff which rises on the south side of Sturgeon bay, the traj) cap may be seen not only to abruptly change its thickness and to pass up over the edges of the slates, but also to send a long tongue- like prolongation within the lat- ter apparently parallel to the bedding. On the south side of Prince's bay, in passing eastward, the trap cap is seen to de- scend across the bedding of the slates to the water's edge; and and at the point which forms the headland of the bay it appears clearly to cut the slates. Southward from the point the trap continues to cut the slates for two or three hundred yards, and then passes up over them and merges with the trap cap once moi'e. The appearance is very much at one place as if there were a large dyke in direct continuity with the trap cap. At the same locality is another dyke distinctly cutting both slates and trap cap. *Op. Git. pp. 42. 46, Til, 80. !).». +Op. Cit. pp. 873.374. 86 HULLIOTIN NO. VIII. On tho north sid(> of Little Trout buy ut a point about N. N. E. of McKollar's point, may be seen a very clear case of an intrusive sill. Tin? sill is al)out four feet thick and is composed of a dark ^'ruy diabas(>. It has boon inj(;ctod alon<.,' a struc- tural plane in an older and tliick(;r sheet of a pirdtish weather- ing trap which presents a marked contrast to the sill. The latter has a dip of probably 10 to the southward. It is at the water's ed cii)) to tlio Aniinikie slates on the south side of Little Trout bay. of trap and slate may bo seen on the south side of the extrem- ity of McKellar's point. Here again the trap mounts over the edge of the fiat slates and then continues as a horizontal sheet resting upon them. There is here, liowever, one interesting point of ditference from the conditions last referred to. The edges of the flat slates do not abut squarely upon the intersect- ing trap, but are sharply bent and broken at the contact, and in this appear to atTord evidence of a fault strain or monocline flexure which preceded the rupture of the slates. (Pig. 2.) On the north side of Big Trout bay near the end of the bay. the great vertical clitTs show, besides the common thick cap of gray trap on the slates, tWo very well marked sills at lower levels and near the shore. ^. .ese are nearly horizontal and lie i.Acicoi.rnc sili.k. 87 parallol willi Iho boddin^', ilie ui)i»or ono l)»'iii<^ Id or IL' foet thick and the lowor out! 4 (;i' "i foet. Thoy aro very distinctly col- \imtiar and very well dotincd l)asaltii' columns of siniill dinion- siotis may be easily removed from the ed^^'t's of thi' she((ts. Both of these sheets are tine point showinsr I lie icl.ilioii of the triip cup tot lie AiiiiiiiUi<; strjitii. intercalated with the slates. This is clearly an injected sill, for it may be seen in the cliff face to pass from one bedding plane, across the edge of the slates above and below, to another hori- zon about 10 feet higher. In the same section there is also a vertical dyke which seems to antedate the horizontal sill. The relations observed are represented diagrammatically in figure three. The relation of the small sill to the thicker mass of trap could not be clearly ascertained. IMAGE EVALUATION TEST TARGET (MT-3) V / O // I ^ ^/# '^^ V (/j A 1.0 I.I 1.25 "■MUM ilM ■ |3 2 43 6 22 12.0 1.8 U 111.6 v} ^ 7^ /} ^;. ■^p ^^i o1 *^ .>^' d? / Photographic Sciences Corporation ^v ^^ iV ^ ^\ ^ '<^ «: 6^ k ^ % n? <^ "9.^ 23 WEST MAIN STREET WEBSTER, NY. )4580 (7U) 872-4503 f^" g 38 BULLETIN NO. Vlll. An intorestinf? case of an intrusive sill is shown in the cliff section on the southeast side of Victoria island. Hero a four- foot sill of gray diabase has been injected aloiijr the plane of contact between the Animikie slates and an earlier .sheet — ' ' _ J ■ ^^■**^^?*«^ — ;■ ^^ i'l t/(il 1/ IJ 1 Lml .iiititt sui FIr. 3. DlrKtiiiiiiiiiitlcsi'clioii near the exfi-enilty of llic point Ix'twccn Hiu' Trout ati willi imdci lyiiit; Aiiliiilkir strata on tlii> .shore opposlto Vii-toriu Isluiul. The filling of minute fissures and cracks in the slate by apophyses from the trap sheet shows the same thing. The slates, then, were hard and brittle and had acquired their characteristic cleavage prior to the advent of the traps. The numerous cases of the trap sheets cutting across the bedding planes, as above recited, also shows the ti'aps to be later than the induration of tl>e slates and the development of the cleav- age. It is clear, tlion. that even if the trap caps ai'e volcanic Hows, they are not contemporaneous with the deposition of the Animikie sediments. If then, they are volcanic tlows at all they come under the second of the two possible suppositious. This implies that they were extravasated as flows on an eroded sur face of the Animikie rocks. The study of the contact lends no countenance to this supposition. An eroded surface has two pronounced features which are absolutely lacking in the surface upon which the trap caps repose. These are (1) the common evidences of sub-aerial weathering and the sculpture of the sur- face, and (2) the accumulation of surface debris. The absence 42 BULLETIN NO VIH. of these features in any single section, or even in several locali- ties, would not be conclusive; but when it is remembered that the nature of the exposures is such, that the unobscured con- tact may be critically examined along hundreds of miles of irregularly winding mesa scarps, the utter failure to find any evidence whatever of erosive action upon the surface covered by the trap caps, becomes full warrant for affirming that the surface has never been exposed to such agencies. The surface of the Animikio slates beneath the trap presents always the characters of a rock freshly ruptured and inveloped immedi- ately in the trappean magma. Thus again the sujiposition that the sheets are surface flows has nothing to sustain it, and all the evidence directly supports tho view that the sheets are intrusive. I THE UPPER CONTACT. In most of the intercalated sheets the evidence of intrusion is so abundant that it is scarcely necessary to go into the de- tails of their upper contact with the enclosing rock. It is suf- ficient to say that it presents no essential difference from the lower contact. The trap involves the sharp angular edges of %e slate, where they have been ruptured by the disturbance: apophyses from the sheets extend (usually for not more than a few inches) into the cracks and fissures of the overlying slates; and angular fragments of the latter ai'e quite frequently found imbedded in the trap. As has-been stated, the opportunities of examining the upper contacts of the trap caps are few, ow- ing to the general removal by erosion of the slates which once rested upon them. In the vicinity of Port Arthur, however, some remarkably fine sections may be very conveniently ex- amined which show remnants of slates still reposing upon the trap caps; and such instances would be doubtless more fre" quently observed, were it not that the routes of travel only oc- casionally traverse the surface of the caps, and that the latter are very commonly covered v,'ith timber. The Port Arthur trap sheet is an extensive one, and not only underlies the town but extends inland for many square miles and skirts the shore of Thunder bay, being found on the points and islands. The av- erage thickness is probably about 50 feet. On the surface of this sheet at several places may be seen patches of slate, ad- hering to it, and frequently sunk down into it. An excellent section is afforded in the railway cutting at the old Canadian Pacific railway station at Water street. Port Arthur. This section is shown in Plate VI. (frontispiece). The upper por- LACCOLITH • SILLS. 43 tionof the trap sheet is very dense and compact, but it rapidly becomes coarser in descendinj^r through the thickness of the stieet. The plane of contact is on the whole, even, but is ra<;- «^ed or step like in detail. th(! stejis being due to the fractur ing of the slates by the invasion of the trap. The nldfcs oi'- altered to the (liMance of orcr a foot abort' the trap. In other sec tions the contact is still more step-like, and blocks of slate may be seen to he sunk down within the trap. The surface of the sheet, in places where it is perfectly evident that it has not been affected by erosion, is peculiarly domed with low flat un- OlVfftaOorfl.iikfSniieiHn- "r^ mm I't-afi Can ipflTan, PimiiHiEnjnrirain'T'iEG! Talus chief I If trap, bleclta. RCHfrffif '/''•a/i shfet T-alU't chief 11/ shalti ^^iss" and ■slaty snnH stone ~' "-:— — \^¥^^;»>V ^Ez.:-:rn=^A •^v" S /^ ake lei'tl l''l;;6. Diasfiiiiiniutic sei'lloii tlironu'li McKay'-^ Moiiiitiiiii. Fort WiUiatn, sliowiiiij the relatiuii or Liicoolitii.' Sills to the Aiihiiikli" sirivtu. Pciile I inch. ^'KU fci'i. duladons. The entire character (Tf the up))er surface of the Port Arthur sheet, which is one of the great caps of the region, although locally it descends to the level of the lake, is strictly analogous to that which may be observed on the projecting surface (a terrace of differential degradation) offered by the intecalated sheet at the Romar Catholic retreat on the face of McKay's mountain at Fort William. This sill is about 12 to 15 feet thick (see fig. 8) and has the same locally domed orundul- 44 JJULI.KTIN NO. VIII. atory surface with patches of th(; overlying slates sunk dowu into it and partially imbedded in it. Tliis domed surface of the trap sheets has been also noted by In^all* in othej* localities. ALTKKATION OF THE ENCLOHINC. ROCKS. The alteration of the Animikie slates by intrusive masses of the same petro*?raphical character and fjeneral dimensions, is very various in its extent. In some cases, as in the remarkable one at Pigeon point, described by Bayley. it amounts to a com- plete fusion of the invaded rocks and the mixture of such fused rock Avith the invading magma, giving rise to peculiar and ex- ceedingly interesting petrographical types. In other cases the slates have only been altered to a hornfels to the extent of a few inches, or at most a few feet from the contact. Sometimes it requires a trained petrographical eye to detect that there has been any alteration whatever. The rocks immediately ad- jacent to the trap sheets, whether at their lower or their upper surface, ai'e, however, always altered to the extent at least of making them recognizable as hornfels. They are hard and dense and fre<]uently resemble somewhat the den.se aphanitic facies of the trap at the contact, so that some geologists have confessed their inability to distinguish between them. This zone has not been subjected to systematic petrographical study, but in the few slides which the writer has examined, it is clear that the clastic structure of the rock has been more or less ob- scured by re-crystallization, and that the rock is characterized by the abundant development of minute pleochroic needles having parallel extinction, and resembling green hornblende but for the latter property. This alteration is sometimes associated with a bleaching of the rock; and in some cases there are suggestions of secondary glass having been formed. But it is not the intention of the writer to discuss the nature of the alteration. It is sufficient for his purpose to be able to state that there is a prevalent alteration of the Animikie slates, both above and below the trap sheets, which is clearly ascribable to their invasion within the slates as ingneous masses. SUMMARY. The argument may be summarized briefly: I— The trap .sheets associated with the Animikie strata are not volcanic flows, because of the combination of the following facts: 'Op. Cit. LACCOLITIC SILLS. 1 n. •1. '). 0. 7. 8. They aro simpl*' f,'eolo«;if'{il units, not a series of over lapping sheets. They arr^ Hat with uniform thickness over areas more than one hundred square miles in extent, and where inclined, the dip is due essentially to faulting and tiltin/iif. There are no pyroelastic rocks a.ssociated with them. They are nev(;r glassy. They are never aniygdaloidal. They exhibit no tlow structure. They have no ropy or wrinkled surface. They have no lava breccia associated with them. They caine in contact with the shites after the latter were hard and brittle and had acquired their cleavage; yet they never repose ujion a surface which has been exposed to sub- aerial weathering. 11. — They are intrusive sills because of the combination of the following facts: 1. They are strictly analogous to the great dykes of the region. {") In their general relations to the adjacent Tocks, and in their field aspect, (h) In that both the upper and lower sides of the sheets have the facies of a dense ay^hanitic roclc. which grades towards the middh' into a coarsely crystalline rock. They have a practically uniform thickness over large areas. The columnar structure extends from lower surface to upper surface, as it does from wall to wall in the dykes. They intersected the strata above and below them after the latter had been hard and brittle. They may be observed in direct continuity with dykes. They pass from one horizon to another. The bottom of the sedimentary strata above them, where- ever it is observable, is a freshly ruptured surface. Apophyses of the trap pass from the main shi^et into tlie cracks of the slate above and below. 10. The trap sheets, particularly at the upper contact, hold in eluded fragments of the overlying slates. 11. They locally alter the slates above and below them. GEOLOGICAL CONSEQUENCES. In the lake Superior region the lowest great division of the Palaeozoic is known to the Minnesota survey as the Taconic system. This system embraces two groups, viz: The Animikie and the Keweenian ^Keweenawan). Between the Animikie and the Keweenian there is an interval of erosion and a consequent 2. 3. 5. 6. 8. 9. 4f') BULLETIN NO. V\l\. unconformity. In the opinion of tlie writer this uncont'orniity represonts a mucli moro important interval than has been com- monly supposed. Tlio slight amount of distiirbance, which alTected tlio Animiki<» roclcs prior to the deposition of the Ke- woonian. leaves the two sets of rocks in nearly parallel bedding; and the fact of uplift and long continued erosion of the; Animikie, prior to the dei)osition of the Keweenian, is only realized by a careful study of the contacts of the latter with the lower rocks. The belief that the two groups of rocks were in stratigraphic continuity, or only separated by a slight break, seems to have led the earlier geologists to regard the trap sheets associated with the Animikie as simply a manifestation of the volcanic activity which prevailed in Keweenian times. The propositions established in this paper tend to empha- size the contrast between the two divisions of the Taconic system. Not only is there a pronounced stratigraphic break between the Animikie and the Keweenian, but the earlier group was deposited in a time of quiescence entirely free from vol- canic disturbances, while during the deposition of the latter, volcanic extravasations were poured out in no sparing .meas- ure. The contrast which is thus presented by the two divisions of the Taconic system is analogous to that which obtains between the two divisions of the Ontarian system in the Archa3an. The lower group of the Ontarian system, the Coutchiching. is a great series of metamorphic sediments free from volcanic rocks;* this is followed by the Keewatin group, in which volcanic rocks and their metamorphic products predominate. The recognition of the trap sheets as laccolitic sills opens up to students of American geology a large and interesting prob- lem of a phase of crust-building which is here barely touched upon. The lateral injection of great sheets of molten matter within the bedding planes of horizontal series of rocks is of course a well known fact. But it is also a problem in physical geology which has not been thoroughly worked out; and the prevalence and importance of the process of lateral intrusion, as a factor in the development of the earth's crust, is far from being sufficiently recognized in geological literature, notwith- standing the impetus which has been given to this line of in- quiry by Gilbert's classic work on the Henry mountains.! *Coniparo. howevpr. N. II. and 11. V. Wlnchell (Iron Ores of Minnesota, pp 11-24) who show ttiiit probably the verniilloii (OoutchichinK) sediments are modifleu ounditlons of older sediments of like origin and cbaracter with those of the Keewatio. tOonipare also N. H. Windiell. Some thoushts on eruptli^e rocks with special refer- once to those of Minnesota, A. A. A. S., 1888, pp. 212-281. laccomtk; sills. 47 ,.* To investigators dpsirons of attiiolcinf^ this important prob- iotu no more successful liold. and no inoro iii)untiant opportun- ities for unobscurod obs(!rvation of fr(»sli rocl^s. could be de- sired than those presented by the northwest coast of lake Su- l)erior. The laccolitic charact<'r of tlic trap ca])s of the region of course implies an extensive erosion anil removal of tlieoncc ovi'r- lyinff strata. How thick this volume of rock origiiuiUy was we have little means of knowing. It seems not improljable, however, that over much of the Animikie region now capped with tliese thick sills, not only was there a very consideral)le thickness of the u])p(M' portions of the Animikie. l)iit also that the Keweenian strata f)ccui)ied the same area. The sills are not only of later age; than the Animikie. but they appear to be of distinctly post- Keweenian age. As has been shown, the sills jiass from the hori^con of the Animikie to the Keweenian. and the same features which prov(> them to be in- trusive in the Animikie show then also to be intrusive in the Keweenian. This is true, not only of the Keweenian of the Canadian side of lake Superior, but also of the Keweenian of the Minnesota coast. While fully recognizing the essentially volcanic character of the Keweenian series the writer has no hesitation in stating his 0/H///0/* that many t)f the heavy sheets of dark diabase or gabbro, whi(;h prevail on the Minnesota coast, particularly in its eastern i)ortion. and which have been described and referred to by former observers as volcanic flows of Keweenian age. are laccolitic sills. The.se have been inject- ed along the bedding planes of the volcanic sheets of the Ke- weenian, just as they have been between the sedimentary planes of the Animikie. These are, therefore, of post- Keweenian age. Many of these sheets are petrographically identical with those of the Animikie province, while others present differences in mineralogical detail and general aspect. The recognition of the laccolitic character of the trap sheets associated with the Animikie rocks, tlnd of their analogues associated with the Keweenian, involves the recognition also of a distinct and important event in the geological history of the lake Superior region. The intrusion of these sills is an event subsequent to the formation of the Animikie and Keween- ian rocks. How much later in age they are we do not yet know, as there are no other palaeozoic rocks in the region to serve as a proximate limit. They may. indeed, be much later than the Taconic, and may possibly correspond in age with the 48 mil.LETlN NO. VIII. jfreat series of Iriij) rocUs ititnuli'd in llu; Siliiriun njcks of Quebec, of which mount Koyal, at Montreal, is a well known in^>tance. In view of the uncertainty as to thoir aj