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 MC909 
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 ii^.^ 
 
 Reprinted from The Journal of Geology, Vol. i, No. 2, 
 February-March, 1893. 
 
 J.j-^^<^c 
 
 =7 yy 
 
 t 
 
 \ 
 
 AN 
 
 Historical Sketch 
 
 OF THE 
 
 Lake Superior Region 
 
 TO 
 
 Cambrian Time 
 
 By C. R. van HISE 
 
 I 
 
 CHICAGO 
 ' D. C. HEATH & CO., Directors 
 
■ 
 
■)• 
 
*!« 
 
 ' 
 
 THE 
 
 JOURNAL OF GEOLOGY 
 
 FEBRUARY- MARCH, iSgj. 
 
 AN HISTORICAL SKETCH OF THE LAKE SUPERIOR 
 RF:GI0N to CAMBRIAN TIME.' 
 
 (with PLATE I.) 
 
 The ancient formations south of Lake Superior may be 
 grouped into five great divisions : the Basement Complex, the 
 Lower Huronian, the Upper Huronian, the Keweenawan, and 
 the Lake Superior Sandstone. These five divisions are separated 
 by unconformities of great magnitude, two of them at least 
 being of the first order. According to the classification adopted 
 by the United States Geological Survey, the Basement Complex 
 is Archcan ; the Lower Huronian, Upper Huronian and Kewee- 
 nawan constitute the Algonkian for this region ; and the Lake 
 Superior Sand.stone is Cambrian. 
 
 The Basenii t Complex. — The characteristic rocks of the 
 Basement Complex are ( i ) light colored granites and 
 gneissoid granites, and (2) dark colored finely foliated or 
 banded gneisses or schists. These are cut by various basic and 
 acid intrusives, many of which are not different from eruptives 
 
 ■In this very general article no attempt will he made to give references to the 
 many authors from whom facts are taken. To give full credit for all information 
 used would require citations from scores of papers. The writer gives a summary 
 of the literature of the Lake Superior Region in Bulletin 86 of the U. S. Geo!. 
 Survey. 
 
 Many of the problems considered have no definite answers as yet. The aim of 
 the article is to give a summary of the very limited knowledge available on a sub- 
 ject that has not before been considered, because the data were not at hand upon , 
 which to base any reliable conclusions. 
 Vol. I. — No. 2. 
 
(;i:OLO(;i( AL MAPOKTHI-: I.AKK SI 
 
 SJIOWINO nil-. CAMHUIAN AXl) CKYS'l 
 
 Compiled IrotnOllKinl MopsolU S.nncI Oiiindit 
 
 S<-rtle 
 o 50 100 
 
PLATE I 
 
 AI.CiOXlUAK 
 
 HAh 1 ' ' Ak ! 
 
 POST AI.IUiNKlAX 
 
 Ca ! 
 
 lii: CAMHUIAN AND CKYSTAI.l.INH HOCKS. 
 
 )iied irotnOll'KMrtI Maps of l' S.nnd (lnii«Jiaii Stirvcvs 
 
 Scnio 
 
 50 100 ISO STAT Ml. 
 
 Ah riici'riqiilill ntllinrili , ,. , . , ,, 
 
 Ah2 riio Miii-:iiwlli' M"iuiiiiiiicr' Iirn //foi-iNi/ VVi'srs 
 
 Ah J riirWi.'ilfjiisiii Vdllrv Slnt<:\ 
 
 A h 4 riic Pi'iiolni- !nni Fi-iiniiii S^/ll,s^^■ 
 
 , „ , ^, i Ah5 T/w St.l,,,,.,.-. .'H.ile.^ 
 
 M L' R ON I A N ^ ^ 5 j-ii^ Chlripewf fcilln- QiKirtrlU'S 
 
 A h7 The llldrlillivrrlmt l!riiiiri<i Srlus(s 
 
 Ah8 -Iiic Ihiri'lmnViiartxiirfi 
 
 Ah9 The S'uii.) (^tuut\>n's 
 
 A h'O ThfAilimiliu'SrrKs 
 
 A h II [''otdtfiSchist^nfCauitJn 
 
"^ 
 
 114 
 
 7///-; J(H'N\AL OF GEOLOGY. 
 
 fouiul in tbt; later scries, with which they arc doubtless in part 
 
 continuous. 
 
 The granites and gneissoid granites are jilaccd together, be- 
 cause between the two are constant gradations. If one speaks 
 accurately and includes among granites only those rocks which 
 are completely massive, the gneissoid granites include the 
 greater part of ihe granitic rocks ; for in large exjmsures it is 
 usually possible to find some evidence of foliation. The granit- 
 oid areas are of greatly varying sizes, runi\ing from small 
 patches to those many miles in diameter. When everywhere 
 surrounded by the schistose division of the Basement Con.plex, 
 they fre(p;ently have oval or ovoid forms. In nearing the outer 
 border of the granitoid areas, the foliation often becomes more 
 and more prominent, and near the edge of an area t..w rock fre- 
 cjuently passes into a well laminated gneiss. 
 
 The schistose rocks include fine grained hornblende -gneisses, 
 mica -gneisses, chlorite -gneisses, and various green schists, for- 
 merly sujjposed to be sedimentary, but now known to be greatly 
 modified basic and acid igneous rocks. These schists have 
 usually a dark green or black color, are strongly foliated, and 
 the variations in strike and dij) of this foliation, within small 
 areas, is very great. Not infrecjuently the schistose rocks are 
 traced by gradations into massive igneous rocks. 
 
 The contacts between the schistose division and the granit- 
 oid division of the Basement Comjilex are usually those of in- 
 trusion, the granitoid rocks being the later. In i)assing from a 
 schistose to a granitoid area, small pegmatitic looking veins of 
 the granite are first found. In going onward these veins beconie 
 more numerous and. after a time, unmistakable dikes of granite 
 ai)pear. which multiply in number and size in aj)proaching the 
 granite area, until the granite is found in great bosses. Here we 
 have i)erhaps a nearly ecjual c|uantity of schistos.: and granitoid 
 rocks, and in this intermediate zone the schists may be found as 
 a mass of blocks within the granite, soinetimes at but -mall dis- 
 tances from their original positions, the whoie having rretiucntly 
 a somewhat ct)ng!omeratic api)earance. However, these jiseudo- 
 
"^ 
 
 ///ST(>A'fCAL Sr-ETClf I.AKK SUPKRfOR REGION. 
 
 "5 
 
 conglomerates, so well describcl by La. /son, grade more or less 
 rapidly on the one hand into the schists, and on the other into 
 the solid gneissoid granite. The complete cjiaiigf may occur 
 within a short distance, or it may take a mile oi more. 
 
 The Hasement Complex is then composed of intricately inter- 
 locking areas of granitoitl rocks and schistose rocks. Moreover, 
 all of these rocks are completely crystalline. None of them 
 show any vmiuistakable evidence of havinii^ been derived from 
 sedimentaries, but many can be traced with gradations into 
 massive rocks, and there, jre the greater jjroportion of them are 
 igneous, if a comjiletely massive granular structure be proof of 
 such an origin. 
 
 "^he Hasement Complex is the most widespread of any of the 
 Lak«. Superior systems, and it tic 'nless runs under all later for- 
 matioi s to a greater or lesser distance. That it is continuous un- 
 der all such formations can not be asserted, for while it was once 
 so, it is j)ossible. perhaps even probable, that in places, as a con- 
 se(juence of sedimentation and folding, the Basement Complex 
 has oeen so deeply buried, that fusion has locally resulted. It 
 is even possible that such fused material is a partial source of the 
 later volcanic eruptions. 
 
 Before the earliest sedimentary rocks were dejjosited, the 
 Basement Complex was subjected to enormous orographic forces, 
 which folded and sheared the rocks in a most intricate manner. 
 Accompanying the great orographic movements, which undoubt- 
 edly occupied a vast period of time, were intrusions of various 
 deep seated igneous rocks, and also doubtless their volcanic 
 e<]uivalents were extruded. Subsei]uent to, and during the oro- 
 graphic mo ements, atmospheric forces were at work. Erosion 
 continued long after the mountain -making folding had ceased, 
 and, for much of the Lake Superior region, reduced the Base- 
 ment Complex nearly to a plain or base level. As evidence of 
 this may be cited the fact that, at the end of the erosion inter- 
 val, the Basement Complex, consisting of differing lithological 
 materials, and therefore having a variable resisting power, did 
 not vary in alti' 'e more than a few hundred feet for long dis- 
 
I 
 
 ,l6 THE JOURNAL OF GEOLOGY. ' 
 
 tances. Whether this denudation extended everywhere deep 
 enough to remove all surface vole ■ .lic material, and to leave only 
 deep seated igneous material, is undetermined. At the beginnmg 
 of the Lower Huronian time, the Basement Complex was, in the 
 Lake Superior region, a universal system. 
 
 The Lower Huronian.— Mi^x the forces of erosion had nearly 
 exhausted themselves, there was the f^rst advance of the sea over 
 the Lake Superior region of which we have any evidence, as a 
 result of which the Lower Huronian was deposited. 
 
 The well-known characteristic rocks of the Lower Huronian. 
 are (i) conglomerates, quartzites, quartz -schists and mica- 
 '.rhists. (2) limestones. (3) various ferruginous schists. (4) baste 
 uid acid eruptives. which occur both as deep seated and as effu- 
 sive rocks. The order given, with the exception of the erup- 
 tives. is the order of age from the base upward. 
 
 The inferior formation is usually a quartzite or a feldspathic 
 quartzite Where metamorpbism has been severe it passes into a 
 quartz -schist, mica -schist or gneiss. The lowest horizon of 
 the formation is in places a coarse conglomerate, and this when 
 metamorphosed may become a conglomerate -schist. This con- 
 glomerate is of two types, depending upon the character of the 
 underlying formation, which is here granitic and there schistic. 
 The limestone formation, when at its maximum, is of very con- 
 siderable thickness. The limestone is magnesian and so very 
 crystalline as to make the name marble appropriate. It fre- 
 quently contains a considerable amount of chert. In places it 
 maybe divided into two horizons, , one of which is nearly pure 
 marble, and the other nearly pure chert. At other times the 
 limestone becomes very siliceous by a mingling of fragmental 
 quartz, while zones of wholly fragmental material may occur. 
 These impure phases are often at the lower part of the limestones. 
 where they may be considered as a transition from the under- 
 lying formation. The formation overlying the limestone is 
 usually known as the iron -bearing member, since it contains all 
 the ore bodies of the Lower Huronian. It has varied aspects, but 
 the different varieties grade into one another )*oth vertically and 
 
 I 
 
HISTORICAL SKETCH— LAKE SUPERIOR REGION. I I ^ 
 
 k 
 
 laterally, so that when one becomes familiar with them, the rocks 
 of the formation may invariably be recognized. Here are included 
 hematitic and magnctitic schists, cherts, jaspers, ferruginous 
 carbonates, and other forms. The formation always differs from 
 the limestone in carrying a very considerable amount of iron, and 
 it differs from the quartzite in being largely, and sometimes 
 wholly, a chemical or organic sediment, rather than a mechani- 
 cal one. 
 
 The three members of the Lower Huronian are not often seen 
 in a single section. This may be due to lack of exposures, but 
 in some case^ is undoubtedly due to the absence of one or more 
 of the formations themselves. 
 
 In the Lower Huronian, basic eruptive rocks are abundant, 
 and locally cover considerable areas. Not infrequently acid 
 eruptives also occur. These eruptives include both contempor- 
 aneous volcanics and subsequent intrusives. If- the Keewatin of 
 Lawson about Rainy Lake and the Lake of the Woods is Lower 
 Hurnian, great granitic masses have been intruded into this 
 series northwest of Lake Superior. 
 
 Equivalent to the Lower Huronian series of the north shore 
 of Lake Huron are placed the following iron-bearing districts : 
 Lower Vermillion, Lower Marquette, Felch Mountain, in large 
 part. Lower Menominee, the cherty limestone formation of the 
 Penokee district ; and probably the Kaministiquia series of 
 Ontario, and the Black River Falls series of Wisconsin. Whether 
 all of these detached basins were once connected by continuous 
 sediments is unknown, but probably they were. 
 
 The fragmental material of the Lower Huronian was derived 
 from the Basement Complex. This fragmental formation is usu- 
 ally thin. This doubtless means that the advance of the sea 
 over the Lake .Superior region was comparatively rapid. The 
 directions from which the Lower Huronion sea entered, and the 
 extent of its trangrcssion. is at present unknown. By certain of 
 the Canadian geologists it is held that the structural break which 
 exists between the Basement Complex and the Lower Huronian, 
 south of Lake Superior and north of Lake Huron, does not exist 
 
Ii8 
 
 THE JOURNAL QP GEOLOGY. 
 
 in the region of Rainv Lake and Lake of the Woods, northwest 
 of Lake Superior. If this conclusion be true, the sea did not 
 advance as far as the Lake of the Woods, this district perhaps 
 being above the ocean, and one of the sources of detritus through- 
 out Lower Huronian time. 
 
 The extent of 1he Lower Huronian deposits is also uncertain. 
 If the series of the districts above placed in the Lower Huronian. 
 are correctly correlated. Lower Huronian basins occurred in vari- 
 ous places over a great triangular area extending from Black 
 River Falls in Wisconsin, to northeastern Minnesota, and thence 
 east to the north shore of Lake Huron. Doubtless Lower 
 Huronian rocks also occur in the great northern region of 
 Canada, and they may have had a much wider original extent 
 than this, but no data are now available to locate such a possible 
 extension. 
 
 Of the original thickness of the Lower Huronian dejwsits we 
 are also ignorant. The present thickness has not been deter- 
 mined south of Lake Sui)erior, but according to Logan, on the 
 north shore of Lake Huron, including the interstratified volcanics, 
 the thickiiess is five thousand feet. 
 
 At the end of Lower Huronian time, the Lake Superior 
 region was raised above the sea, folded, and subjected to erosion. 
 The orographic movements of this time were very severe, closely 
 crumpling in places the rocks of the Lower Huronian, and induc- 
 ing in them in many places a schistose structure. In other locali- 
 ties, away from the axes of great disturbance, the Lower 
 Huronian rocks were but gently tilted, as is shown by the small 
 discordance in places between them and the succeeding scries. 
 In certain localities the areas of great disturbance are but a short 
 distance from those of comi)arative (juiet. The denudation was 
 deep enough to wholly remove the entire series over wide areas, 
 and to cut to unknown depths into the Basement Comi)lex itself. 
 As has been stated, the Lower Huronian has an estimated thick- 
 ness of about one mile on the north shore of Lake Huron, and 
 in different localities varies from this thickness to entire absence, 
 depending mainly upon the differing deundation. This variabil- 
 
HISTORICAL SKETCH^LAKE SUPERIOR REGION. II9 
 
 J 
 
 ity may possibly be due in part to highlands of the Basement 
 Complex, which were not covered by the Lower Huronian sea 
 until the period was well advanced. Of the extent of the series 
 at the end of the erosion preceding Upper Huronian deposition, 
 little has been determined, since later erosions have undoubtedly 
 removed large areas of the series, and therefore its present dis- 
 tribution is not a safe guide to its distribution at the close of the 
 erosion interval referred to. 
 
 The Upper Huronian. — At the close of the long period of 
 erosion which followed the Lower Huronian deposition, the 
 water once more advanced upon the Lake Superior region, and 
 the Upper Huronian series was deposited. 
 
 Lithologically this series consists of conglomerates, quartzites, 
 graywackes, graywacke-slates, shales, mica-schists, ferruginous 
 slates, cherts, jaspers, ferruginous schists and igneous rocks, 
 including both lava flows and volcanic fragmentals, as well as 
 basic and acid intrusives. The series, as a whole, is very much 
 less crystalline than the Lower Huronian, although locally the 
 shales and graywackes l.ave been transformed into mica-schists, 
 and even into gneisses. 
 
 .''* The Upper Huronian immediately about Lake Superior is 
 divisible into three formations, a lower slate, an iron-bearing for- 
 mation, and an upper slate, the basis of separation being that of 
 mechanical and non-mechanical detritus. The inferior formation 
 is mainly a cjuartzose slate or shale, but locally it [)asses into a 
 quartzite, while the basal horizon is frequently a conglomerate. 
 The nature of this conglomerate varies greatly, depending upon 
 the character of the underlying formation, which, in some areas, 
 is the Basement Complex, and in others the Lower Huronian. 
 In the first case the slates may rest upon the gneissoid granite, 
 upon the schis :, or upon the junction of the two. The basal 
 conglomerate corresponds in its character, being a recomposed 
 granite or granite-coaglomcrate, a recomposed schist or schist 
 conglomerate, or finally a combination of the two. 
 
 When the lowest member of the Uj)pcr Huronian rests upon 
 the Lower Huronian series, the underlying lOrniation may be 
 
120 
 
 THE JOURNAL OF GEOLOGY. 
 
 any one of the three formations of the Lower Huronian. As a 
 conse(}uence the basal conglomerate may consist mainly of the 
 fragments of any one of these formations, or of all of them 
 together. Not infrequently detritus, derived from the Basement 
 Complex, is mingled with that of Lower Huronian origin. How- 
 ever, as a consequence of the resistant character of the jaspery 
 iron-bearing formation of the Lower Huronian and of mining 
 operations, the discovered contacts are most frequently between 
 the Upper Huronian and this iron-bearing formation. In the 
 basal conglomerate or recomposed rock at these points, the 
 characteristic fragments are chert, jasper, and other ferruginous 
 materials, and it is locally so rich in iron as to bear ore-bodies. 
 The uppermost horizon of the lower slate of the Upper Huronian 
 in the Penokee district is a pure, persistent layer of quartzite. 
 The central mass of the formation is a graywacke or graywacke- 
 slate, passing in places into a shale or sandstone. 
 
 Above the lower slate is the iron-bearing member, consisting 
 of variou'= ferruginous rocks, including cherts, jaspers, magnetite- 
 actinolite-schists, iron ores, and ferruginous carbonates. It has 
 been shown that all these varieties have been mainly derived 
 directly or indirectly by transformation from an original lean, 
 iron-bearing carbonate, which was of chemical or organic origin, 
 or a combination of both. Mingled with these non-mechanical 
 sediments is a greater or lesser quantity of mechanical detritus. 
 
 Above the iron-bearing formation is the upper slate forma- 
 tion. This is mainly composed of shales frequently carbon- 
 aceous or graphitic, slates, graywackes and mica-schists, often 
 garnetiferous and staurolitic. The mica-schists are usually 
 toward the upper part of the formation. The stages of the 
 transformation between these crystalline rocks and plainly 
 fragmental detritus have been somewhat fully made out . 
 
 The lower slate formation is of variable thickness, but is 
 usually less than a thousand feet. The iron-bearing formation is 
 also of very variable thickness, its maximum being perhaps about 
 the same as that of the lower slate, and from this it varies to dis- 
 appearance, the horizon being usually represented, however, by 
 
 I 
 I 
 
HISTORICAL SKETCH— LAKE SUPERIOR REGION. 121 
 
 I 
 I 
 
 carbonaceous and ferruginous shales and slates. The upper 
 slate formation includes the great mass of the Upper Huronian 
 series. Its maximum thickness is more than ten thousand feet. 
 
 In certain areas, during Upper Huronian time, there was 
 great volcanic activity, as a result of which, peculiar formations 
 were piled up, wholly different from any of the ordinary mem- 
 bers of the series. Also this volcanic activity greatly disturbed 
 the regular succession, so that for each of the volcanic districts 
 an independent succession exists, the sedimentary and volcanic 
 formations being intir>iately interlaminated. The two areas 
 which are best known are the Michigamme iron district north 
 of Crystal Falls and the east end of the Penokee district. 
 Similar volcanics also occur in the Marquette district. In 
 the Michigamme iron district is an extensive area of green- 
 stones, greenstone -conglomerates, agglomerates and surface 
 lava flows, many of which are amygdaloidal. In the Penokee 
 district the materials are almost identical. The typical suc- 
 cession for this district extends in unbroken order for fifty 
 miles or more, but east of Sunday L?,ke this is suddenly dis- 
 turbed by the appearance of the volcanics. The character of the 
 rocks and their order soon becomes so different that if one were 
 not able to trace the change from one into the other, there would 
 be a great temptation to regard the part of the series bearing 
 volcanics earlier than or later than the Penokee series proper. 
 But the continuity of the two cannot be doubted. Thus this 
 occurrence well illustrates that lithological character in pre- 
 Cambrian, as in post-Cambrian time is no certain guide as to 
 relative age. Finally, associated with the Lake Superior Upper 
 Huronian rocks are many later intrusive dikes and interbedded 
 sills, chiefly diabases, gabbros and diorites, but local granitic 
 intrusives also occur, particularly in the Felch Mountain and 
 Crystal Falls districts, and possibly also in the Menominee 
 district. 
 
 The typical districts in which the Upper Huronian series can 
 be best studied are the Penokee, Marquette, Mesabi and Animikie. 
 Remote from the Lake Superior region proper, the rock series 
 
122 
 
 THE JOURNAL OF GEOLOGY. 
 
 which arc correlated with the Upper Huronian have not the same 
 successions of formations as in these districts. The Upper 
 Huronian north of Lake Huron has a set of formations which 
 can not be correllated with the formations above given ; the same 
 is uue of other series to the south which arc here placed. The 
 position of these latter as a part of the Upper Huronian must 
 not be considered as a question finally determined, but rather as 
 representing the probability, from the weight of evidence at the 
 present time. It can not be expected that in a great geological 
 basin the same subordinate succession of formations will be 
 everywhere found. 
 
 However, for the present, regarding all these series as Upper 
 Huronian, this is the most widespread of the Lake Superior pre- 
 Cambrian sedimentary series. It includes a great area, extending 
 from the Sioux quartzites of Dakota on the southwest, to the 
 Huronian rocks north of Lake Huron on the east, and thence far 
 to the north, and from Lake Huron to the Animikie series of the 
 National Boundary west of Lake Superior. Within this area 
 are included the major portion of the Baraboo quartzites of 
 Wisconsin; the major portion of the large area in the Upper 
 Peninsula of Michigan, the eastern arms of which are the Menom- 
 inee, Felch Mountain, and Marquette iron-bearing districts ; the 
 greater part of the Penokee-Gogebic iron-bearing series of 
 Michigan and Wisconsin ; the Chippewa quartzites of Wisconsin ; 
 St. Louis slates of Minnesota including the newly developed 
 Mesabi range of Minnesota, and the i^nimikie scries of Thunder 
 Bay, Lake Superior and its westward extension. That most, and 
 perhaps all of these areas were once connected, there can be no 
 reasonable doubt. 
 
 This broad semicircular zone of Upper Huronian rocks, extend- 
 ing from the National Boundary west of Lake Superior through 
 Ontario, Minnesota, Michigan and Wisconsin, to the north 
 Channel of Lake Huron, and thence north to the east side of James 
 Bay, suggests that the transgression of the sea was from the 
 south and east, and that the source of the mechanical detritus is 
 the great expanse of so-called Laurentian rocks west of Hudson 
 
 i. 
 
HISTORICAL SKETCH—LAKE SUPERIOR REGION. 1 23 
 
 I 
 
 Bay and north of Lake Superior. How far the sea transgressed 
 over this aieri, and whether it also advanced toward it from the 
 north and west, is unknown. It is probable as the sea advanced 
 from the south, that the great mass of fragmental detritus, mak- 
 ing up the Baraboo and Sioux quartzites, was laid down before 
 the sea had transgressed to what is now the north shore of Lake 
 Superior, and thus would be explained the discrepancy in the 
 parallelism of formation between the Sioux quartzites, Baraboo 
 (juartzites, etc., and the districts of Upper Huronian rocks adja- 
 cent to Lake Superior. 
 
 In this case the advancing ocean was perhaps making its 
 progress by cutting a terrace (juite as much as by subsidence. 
 However, there is reason to believe that the area included within 
 the west end of the Lake Superior Basin, /. e., from the Animikie 
 series to the Mesabi range, and thence to the Penokee series was 
 submerged practically at the same time. For here we have three 
 great formations of like character in identical order. The lowest 
 formation, the quartzite and quartz-sl?te with conglomerates 
 derived from the Basement Complex a id the Lower Huronian, 
 are the first deposit of the advancing sea. After this came a 
 deepening of the water, when the calcareous and ferruginous 
 formation, now constituting the iron-bearing member, was laid 
 down. Then perhaps as a consequence of the upbuilding of this 
 formation, came a shallowing of the water and the deposition of 
 the great thickness of clayey sediments of the Upper Huronian. 
 Since the last formation must have been deposited in shallow 
 water, and yet is of great thickness, the bed of the ocean was 
 probably subsiding during the remainder of Upper Huronian 
 time. 
 
 At the end of the deposition of the Upper Huronian rocks, 
 the Lake Superior region rose above the sea, and the atmospheric 
 forces once more set to work. The orographic movement follow- 
 ing the Upper Huronian, like that following the Lower Huronian, 
 was locally intense, but in general the folding was of a gentle 
 character. Along narrow axes the plications were so severe as 
 to give the Upper Huronian rocks a foliated structure and com- 
 
124 
 
 THE JOURNAL OF GEOLOGY. 
 
 pletely crystalline schistose or gneissic character, but for the 
 most part the changes in the Upper Huronian rocks are those of 
 cementation and metasomatism. As with the Lower Huronian 
 areas of intense plication, they are sometimes but short distances 
 from those in which the rocks have been .merely tilted. ■ 
 
 How deep the Ujjper Huronian denudation went it is impos- 
 sible to say. We only know that at a maximum, the Ujiper 
 Huronian rocks are now 13,000 feet thick, and in certain other 
 places are entirely absent, the higher members disappearing first 
 and the lower members last. Thus the difference of the Upper 
 Huronian denudation is measured by 13,000 feet. To this must 
 be added the unknown thickness of the Upper Huronian rocks, 
 which have been wholly swept away, and the thickness of the 
 Lower Huronian and Basement Complex, which were cut at this 
 time. The thickness represented by these three elements is 
 unknown, but it is probably great. 
 
 Of the outer limits of the Upper Huronian transgression, we 
 are as ignorant as of the jireceding ones, but certain it is that it 
 had an extent to the outer areas mentioned as belonging to this 
 series. Beyond these limits no knowledge is available. The 
 original extent to the east, south and west of the Upper Huronian 
 will pi-obably ne'C^er be. determined, s-ince the ancient rocks are 
 covered by the Cambrian and post-Cambrian sediments Whether 
 the transgression extended over the Great Northern area of 
 Canada to the Paleozoic deposits will doubtless be ascertained 
 when th's vast region is studied in detail. 
 
 The Keiveenmvan. — Again a change of conditions occurred, 
 and a great flood of basic volcanics. in beds of enormous thick- 
 ness were poured out. Later these were followed by more thinly 
 bedded volcanics. At about the same time a portion, at least, of 
 the Lake Superior region became immersed in the sea, since in 
 places the basement lavas of the Keweenawan are interstratificd 
 with sandstone and conglomerates. 
 
 The Keweenaw series is composed lithologically of gabbros, 
 diabases, porphyrites, amygdaloids, felsites, quartz-porphyries, 
 etc., and of sandstones and conglomerates. The basic and acid 
 
HISTORICAL SKETCH-LAKE SUPERIOR REGIOW 12$ 
 
 rocks constituting the scries are mainly surface flows. The gab- 
 bro flows are often of immense thickness. The diabase flows 
 are usually much thinner, and frecjuently pass in their upper parts 
 into porphyrites and amygdaloids. Many flows are porphyritic 
 or amygdaloidal throughout. The beds of quartz-porphyry and 
 felsite are abundant in certain districts, bvit usually have no great 
 lateral extent, but while a single flow may be traced but a little 
 way, frequently a grouj) of flows of the same general character 
 may have a great extent and thickness. But even the groups of 
 flows cannot be regarded as general formations for the whole of 
 the Lake Superior basin. 
 
 Since the number and thickness df the volcanic beds as well 
 as the detritals vary greatly, the Keweenaw series as a whole 
 is widely variable in different districts in its character and thick- 
 ness. Structurally, Irving has divided the series into two parts, 
 a lower division, in which eruptives are present, and an upper 
 division, in which eruptives are absent. In any one section of 
 the Keweenawan, at the lower part of the lower division, are 
 generally found numerous volcanic flows, with few or no detrital 
 beds. In passing toward the middle of the series the sandstones 
 and conglomerates become more and more numerous and of 
 greater thickness. Still higher the sandstones and conglomerates 
 become predominant, and finally volcanic products disappear, 
 the upper ten or fifteen thousand fe'et of the Keweenaw series 
 being wholly composed of mechanical detritus. A given detrital 
 bed varies from a mere seam of narrow local extent to thick beds 
 of sandstone and conglomerate, one of which has been traced, 
 by Marvine for more than one hundred miles. The most gen- 
 eral detrital formation is the upper sandstone and conglomerate. 
 The Keweenawan rocks extend about the entire area of the 
 Lake Superior basin. They appear upon the east shore of Lake 
 Superior, cover a large area of Keweenaw Point, northern Wis- 
 consin, eastern and northeastern Minnesota, and a great area 
 about Lake Nipigon. A similar set of volcanics, occupying a 
 like stratigraphical position, is also known adjacent to Hudson 
 Bay, and this may be a contemporaneous series. 
 
126 
 
 THE JOURNAL OF GEOLOGY. 
 
 The KcwcenawcMi is the thickest of the series about Lake 
 Superior, its maximum beinjr estimated by Irvin^r at the Montreal 
 river to be fifty thousand feet. From this thickness it varies to 
 nothing. This vast cjuantity of material does not, however, of 
 necessity mark a jjeriod longer or perhaps even as long as the 
 Lower Huronian or Upper Huronian, for the greater part of it 
 is of igneous origin. The lava fiows in their e.xtent and thick- 
 ness are to be conijjared with the great volcanic plateaux of the 
 far West, rathe*- than with local volcanoes such as Vesuvius, or 
 the local volcanoes of the Upper Huronian and Lower Huronian. 
 Associated with the lavas no volcanic fragmcntal material has 
 been as yet discovered. 
 
 The source of the lavat: of the Kewecnawan is beyond the 
 scope of this paper. It was, however, suggested that the fusion 
 of a portion of the Basement Complex, and even Lower Huron- 
 ian. may have in part jjroduced the deejj-seated magmas, the 
 extrusion of which produced the Kewecnawan lavas. 
 
 In large measure the sandstones and conglomerates derived 
 their materials from the volcanics of the series, but a lesser (|uan- 
 tity came from earlier series. This latter is jjarticularly true of 
 the great detrital formation constituting the topmost member of 
 the Kewecnawan. Partly because fragments derived from the fel- 
 sites and porphyries are more resistant than those frcin the basic 
 rocks, acid pebbles are relatively abundant in the conglomerates. 
 
 The fact that erosion was contemi^oraneous with erui)tion for 
 much of Kewecnawan time is to be noted. Certainly, when the 
 period was well inaugurated, most of the Lake Superior basin 
 was normally below the sea or near tide water. Many of the 
 eruptions may have been sub-atiucous. Here and there volcanic 
 masses of such magnitude were built u|) as to rise above the 
 water, and ujjon such areas, the sea at the base, and vhe air and 
 rain above, immediately began their course of destruction. The 
 acid and more viscous lavas may have formed the more i)romi- 
 nent elevations, and thus the attack was here more vigorous. 
 This may partly explain the predominance of the acid pebbles 
 in the conglomerates. 
 
HISTORICAL SKETCH -LAKE SUPERIOR REGION. I 27 
 
 This j^rcat volcanic period was doublcss one of unstable 
 equilibrium, the lithosphere fallinj^ here anil rising there. One 
 of the final movements was the production of the Lake Superior 
 synclinal. This synclinal movement affects not only the 
 Keweenawan rocks, but the lower series, and in areas in which 
 the unconformity between the Upjier Huronian and the Kewee- 
 nawan is not grjat, there is such a likeness in strike and dip of 
 the two series as to suji^gest, at first, that the two are conform- 
 able. It is only as the contacts between them are followed for 
 some distance, and the Keweenawan is seen to be now in contact 
 with one member of the Upper Huronian, and now with another, 
 that it is perceived that between the two there is an uncon- 
 formity. 
 
 What proportion of the Keweenawan had accumulated before 
 this Lake Superior synclinal began it is impossible to say. Pos- 
 sibly somewhere near the center of the Lake Superior I asin were 
 the larger foci, from which the great extrusions of lava occurred, 
 and here a simultaneous sinking went on, such as is usual as a 
 result of the upbuilding of a mountainous mass of volcanic 
 material. This suggestion, if true, would also jiartly explain the 
 apparent absence of volcanic fragmental material which naturally 
 would accumulate near these foci. 
 
 Nowhere are the Keweenawan rocks so closely folded as to 
 give them a schistose structure or a metamorphic character. 
 Their induration is almost wholly a process of cementation. 
 
 The Cambrian Transgression. — At the close of Keweenawan 
 deposition the Lake Superior region was again raised above the 
 sea, and the pre-Cambrian erosion continued until the enormous 
 thickness of Keweenawan deposits was wholly truncated. What 
 must have been mighty mountains were reduced to mere stumps, 
 or to base level. Following this denudation, the sea once 
 more transgressed upon the land, and the horizontal Lake Su- 
 perior sandstone was deposited. It now occupies many of the 
 bays about Lake Superior. It once was much thicker, and per- 
 haps covered all but the highest points of land. Certainly it or 
 an overlying formation once was at least one thousand feet higher 
 
 I 
 
128 
 
 THE JOURNAL OF GEOLOGY. 
 
 than the level of Lake Superior, but it has since been almost 
 completely removed, so that it occurs only in patches within the 
 depressions of the older rocKS. 
 
 Since Cambrian time no important orographic movements 
 nor outbursts of volcanic material have occurred in the Lake 
 Superior region, consecjuently the rocks have received little sub- 
 se(|ucnt alteration. To these facts is due the possibility of out- 
 lining the prc-Cambrian history of this area with greater fullness 
 than has been done in areas in which later disturbances have 
 obscured the early history. 
 
 C. R. Van Hise. 
 
 I