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Tous les autres exemplaires originaux sont f ilm«s en comm^ n?ant par la premiere page qui comporte une empreinte d'imprsssion ou d'illustration et en terminant par la derniere page qui comporte une telle empr^'nte. Un des symboles suivants apparattra sur la derniire image de cheque microfiche, selon le cas: le symbols — ► signifie "A SUIVRE", le symbols V signifie "FIN". Les cartes, planches, tableaux, etc.. peuvent §tre film6s d des taux de reduction diffdrents. Lorsque le document est trop grand pour dtre reproduit en un seul clich6. il est f ilmd d partir de Tangle supArieur gauche, de gauche d droite. et de haut en bas, en prenant le nombre d'images n6cessaire. Les diagrammes suivants illus'trent la m6thode. 1 2 3 32X 1 2 3 4 5 6 t ..1 v'^^*-^*^^ /■ i«* iViJI [From The American Geologist, March, 1891.] PETROGRAPHICAL DIFFERENTIATION OF CER- TAIN DYKES OF THE RAINY LAKE REGIOW. By Andrew C. Lawson, with analyses by F. T. Shutt, M. A., F. C. I. Conininnicatloii No. 2. At the Toronto meeting of the A. A. A. S. the writers sub- mitted a paper in abstract bearing the above title. The material for the full paper was not at the time of the meeting complete, •». *..; ■ i 154 The American Geologist. March, 1891 and various vicissitudes have since then interfered with the work. However, as some of the facts observed are of interest it is pro- posed to give them here without attempting to elaborate them so fully as was originally contemplated. The dykes of the Rainy Lake region shov,' in a marked degree that variation from a fine texture at the dyke walls to a coai'se texture in the middle of tbe dyke, which is more or less charac- teristic of dykes the world over. In the abstract referred to it was stated that " On examination of the dykes in question, it be- came apparent that this variation in the physical appearance of the dykes is not simply one of texture or degree of coarseness of the constituent minerals, but it is rather the incidental concomi- tant of important structural, tMneralogical and chemical variations which appear verj' constantly in the same way in different dykes." It will be the object of the present paper to establish this general statement by giving some account of the facts upon which it is based. A dyke about 150 feet wide which traverses Stop island on the south side of Kainy lake, in which the variations alluded to are strongly accentuated, will be first desci'ibed somewhat in de- tail. From this dyke a series of four specimens was taken, viz. : I at the contact with the dyke wall. II at four feet from the contact. III at fifteen feet from the contact. IV at seventy-five fe'>^ from the contact (middle of dyke). Textural variation. — To .. unaided eye there is apparent a very distinct gradation in texture from that of an aphanitic rock at the contact to that of a coarse gabbro or diorite in the middle of the dyke. The gradation is rapid in the first four feet, less ao from four feet to fifteen, and scarcely perceptible from fifteen feet to the middle of the dyke ; the rock on which the observations were made in the field presenting a continuous, clean, fresh, glac- iated surface. In order to arrive at some definite information regarding the gradation in texture, careful measurements were made of the constituent minerals in thin sections of the different specimens. The following are the results of these measurements : I Ground mass. ^ Pyroxene — .argest grains 0315 mm Ave. diameter 030 mm Feldspar in slender needles. Ave. size 004 X .053 mm *. ; * ' •' : •• • • : •• •'♦ •/. .•!.••: : * : ;;;•.. •• •. •.•.••/.•**..•'••::'*:*'••/.*•. ••• .•• Petrograpkical Differentiation of certain dykes. 155 Magnetite, ave. diameter 0147 mm Porpliyritic crystals. Pyroxene, largest polysomatlc grain 1. 1:,M) mm Feldspar, largest lath-shaped crystal ()")<) X .. ')()() mm II Pyroxene, approx. ave. diameter 840 mm Feldspar, " " size noo X .or»t) mm Magnetite, " " diameter 12(5 mm III Pyroxene in polysomatlc masses, ave. diameter. . .. ;.'.000 mm Feldspar, lath-shaped crystals, largest 350 X .100 ram Magnetite, few scattered grains, ave. diam 700 mm Quartz, ave. diameter 050 mm IV Pyroxene, much altered to hornblende, larger gr. . 1.000 mm Feldspar, ave. size of larger grains 3.000 mm Magnetite, few large grains, diam 700 mm Quartz, larger grains 840 mm Structural variation. — The specimen taken at the conta(!t with the dyke walls (I) appears in section as a porphyrite. The ground mass is a fine ophitic felt work of plagioclase and greenish yellow pyroxene, with viridite thickly studded with granules of magnetite, all of the latter belonging probably to the final con- solidation of the magma. The porphyritic constituents are in the order of their generation (1) Plagioclase in lath-shaped crystals, -■^"---■^^-^--'■■--'^•""-'"^--''^""-^-'- Fio. 1.-"- -—'- '--^^■■--—- ■ Stop Island dyke. — Section of dyke-rock at contact with dyko wall. X38. a. polysomatlc augite; f. plagioclase. Illustrating porphyrite structure. .56 The American Geologist. March, 1801 either in distinctly isolated individuals or aggregated together in irregularly radiating clusters ; (2) Augite, in rounded or bleb-like, colorless poly somatic masses. The plagioclase is occasionally in- cluded in the augite. The microscopic aspect of this portion of the dyke is shown in fig. 1 . In the specimen taken only four feet from the last, i. e. , four feet from the dyke wall, the structure of the rock presents a marked contrast to that just described. There is no ground mass, and the section presents the character of a typical ophitic diabase as shown in fig. 2. <^^f^^^^f^ Fig. 2. Stop Island dyke. — Section of dyke-rock four feet from dyke wall. X38. a. Augite; f. plagioclase; h. hornblende; m. magnetite. Illus- trating ophitic or diabase structure. In this portion of the dyke the dominant minerals are polj^so- matic augite and green hornblende. The latter mineral is in part derived from the augite, while part of it presents no evidence of such derivation and maj' be original. These two minerals occur in large interlocking masses in which are embedded beautifully sharp idiomorphic ciystals of plagioclase and irregulnr grains of magnetite. It seems probable fi-om a careful inspection of the ; Petrograpliical Differentiation of certain dykes. 167 reliitive attiiude of the constituents that a portion «)f the aujiite is of earlier generation than the rest, and earlier than such portion of the hornblende as may be original. The rounded contours of the fresher masses of pol^'somatic augite suggest analogy with the rounded porphyritic masses shown in fig. 1. In addition to the idiomorphic plagioclase there is a subordinate proportion of plag- ioclase which shows no crystallographie boundaries. Occasionally V iraall grain of quartz may be detected. At fifteen feet from the dyke walls the structure is still ophitic. Augite is the dominant mineral and appears to be of two generations, (1) large, irregu- larly bounded polysomatic masses and idiomorphic crystals, (2) allotriomorphic, interstitial between the idiomorphic plagio- clase. Quartz is sparingl}- present, and magnetite is in large, irregularl}' scattered grains, some of it allotriomorphicall}' devel- oped about the idiomorphic augite. In the middle of the dyke the structure is entirel}' different from cither that of a porphyrite or of an ophitic diabase. It is the tj'pe of structure characteristic of granite, gal)bro, or diorite. All the important constituent minerals interfere with one another, and the only idiomorphic crystals are those of accessory minerals such as apatite. The aspect of a section of this part of the dyke is shown in tlic drawing, fig. 3. Quartz is abundant, and the augite appears to be entirely replaced by horn'olendo, so that the rock would be classed with the (piartz-gabbros or (juartz-diorites according as the hornblende is secondary or original. Considereil simply as a hand specimen it is best termed, probably, a uralitic quartz-gabbro. MineraJogical variation. — The most important mineralogical variation observable in the series of speciniens taken across the d3"ke is the passage from a quartzless rock at the dyke wall to a quart^ose one in the middle of the dyke. No quartz can be de- tected at the side of the dyke. At four feet from the side quartz may be observed in occasional grains, forming an exciHHlingly small proportion of the constituents ; at fifteen feet it is somewhat more al)undant, and in the middle of the dyke (juartz is a promi- nent constituent of the rock. Another important change in the mineralogical composition of the rock is the encroachment of hornblende upon the augite as one passes from the dyke walls, and the final complete replacement of the augite in the middle of 158 The American Geoloylf<t. March, 1891 the dyke. This change, even t.hon<;h it he fine in great part to pju-amorphiHin, testifies to an important variation in the cliaraeter of the rock developed from tlie same magma at various distances from tiie dyice walls. The angite nearer the middle of the dyke would appear to be much more susceptible of paramorphic change than that near the dyke walls. The middle part of the dyke is also richer in accessory minerals such as apatite, biotite, epidote, and leu(!oxene than the lateral parts. Chemical variation. — ^he chemical variations observed in this dyke will be gathered from a consideration of the following table of analyses of the specimens above referred to : Stop Island dyke. I. II. III. IV. SiO, 47.83 1 47.08 47.84 57.. 50 Fo,().,fFeO 4.57 0.72 5.07 AI,0, 3().r28 25.40 23.44 CaO (i.7;.' 8.44 5.02 MixO 4.;5rJ .5.25 2.70 KjO trace . .(50 .45 Na,() 1.30 2.55 i 2.01 >%<>« 2.H) .94 2.02 l-usson ig. 2.05 2.53 2.25 {)<i.2() S.OfiO 100.27 101.12 Sp. g. 3.028 3.080 2.850 These analyses show a remarkable increase in the propor tion of silica in the middle of the dyke over that in the lateral parts. The difference in silica content of about 10 per cent is sufficient to separate the specimens into two distinct rock species according to current methods of classification. The difference is in keeping with the quartzose character of the middle of the dyke as compared with the quartzless character at the side, and also harmonizes with the difference in specific gravities given in the table. Thus in half the space of a sharpl}' defined dj'ke onl}' 150 feet wide our study reveals variations in all of those characters which we make use of in the description and classification of rocks. Totally distinct types of texture, structure and composition belong to tlie same geological unit mass. This fact suggests an interest- ing commentary upon our system of rock classification. Is such classification in cases like the present, or even generally, anything Petrographival Differentiation of certain dykes. 151) more thiin n cliisHifk-sition of hand specuneiis ? Of wluit philo- sophic or geological value is a .claMsificatioii of specimens into different species and types when they may all be one and the same rock crystallized from the same magma within a few feet of one another. The geologist who knows his rocks in the field as well ap in the laboratory finds such classifications ver}' little expressive of geological truth. But the differentiation in character of this <lyke rock suggests other matters tlian a criticism of classification, namely, a consideration of the conditions under which such difi'er- «ntiation was developed from a common magma. From the nature of the case we are able to form fairh satisfactory conceptions as to two conditions which are commonly regarded as having a para- Pio. 3. Stop Island dyke. — Section of dyke-rock seventy-five feet from dyke wall (middle of dyke). X38. f. plagioclase; h. hornblende; q. quartz; m. magnetite. Illustrating allotriomorpliie-granular or granitic structure. j. mount influence upon the solidification of rock from magma. These are (1) the relative pressure, and (2) the relative rate of oooling under which the different parts of the magma solidified. The magma being confined between parallel sharply-cut fissure -walls may fairly be assumed to have been under the same con- stant hydrostatic pressure in an}' given horizontal plane during the 160 77*d American Geoloyist. March, 1801 time of its solidification. It may also be fairly assumorl that those portions of the maj^inii adjacent to the fissure walls cooled more rapidly and solidified earlier than did the middle portion. Thus although wc have in current petrographical literature numer- ous references to great pressure as one of the chief causes of the development of the coarser texture and granitic structure of the plutonic rocks as compared with that manifested by rocks which soli<lif3' at or near the surface, the present case seems to establish the fact that both types of rock structure may be developed under one and the same pressure. Difference in pressure under which magmas solidify is, therefore, probably, not so important a cause of the difference in structure; and texture of rocks as is generally supposed. On the other hand we have in the case imder con- sideration strong presumptive evidence that the rate of cooling which must have been rapid at the sides and slow in the middle, exercised the controlling influence over the character of the rock developed from the magma in any given part. With regard to the conditions which determined the chemical and mineralogical differ- entiation of the dyke rock very little can be definitely affirmed. It seems probable, however, that the explanation lies in the earlier separation of the more basic minerals accompanied b}' a trans- ference of more acid residues (or solvents) to the middle portions, which transference was facilitated by the gradual solidification of the magma from the dyke walls toward the middle, and by the movement of the water constituent of the magma towards the middle. The water of the msigma, so long as the latter remained li(iuid, would have a tendency to escape to the surface. This tendency, taken with the tendency of the higher portions of the dyke to solidify more rapidly than the deeper-seated portions would create a current obliquelj- through the magma, upward and inward from both sides. This current would aid in the transfer- ence from the sides to the middle of the more acid portions of the magmn from which the more basic had separated out. Numerous other dykes have been examined with the same gen- eral result as that arrived at by a studj' of Stop Island dyke. In none of these, however, was the differentiation in character found to De quite so strongly accentuated as in the Stop Island dyke. A series of specimens taken across the dyke which cuts the south- east shore of White-fish bay, and which is referred to in former f PetTographlml Dlferentiation of certain dykes. 161 notes as tlu, White-fish Bay dyke, was analyzed witli the foUowhiff results : * Wliito-lislj I{ay dyke. Fe^Og-fFeO CaO MgO Na^O Loss on Ig Sp. g. I near contact with dyke wall. II six feet from contact. Til thirty feet from contact. IV sixty feet from contact (middle of dyke). In this dyke the gradation in texture is as pronounced as in the Stop Island dyke but the differentiation of structure is not so marked. In I the ground mass has the character of a fine-drained ophitic diabase and the porphyritic constituents present no -reat contrast in size to those of later generation which have crys- tallized around them ; and in IV the ophitic structure is not en- tirely replaced by the granular. This dyke is noteworthy for the abundance of hypersthene which is present near the dyke walls This hypersthene is a porphyritic constituent, and has well defined crystallographic form. It has not been observed in specimens from other portions of the dyke and its occurrence recalls the similar occurrence of enstatite in the Jack-fish Lake dyke and in the Rat-root Bay dyke which hps been noted in a former paper There is as in the Stop Island dyke a regular increase in tiie pro- portion of quartz in passing from the dyke walls to the middle and in the latter part of the dyke the augite is entirely replaced by hornblende. The analyses of this dyke rock and of the Stop Island dyke rock show throughout an unusually high percentao-e of alumina. " A dyke sixty-five feet wide cutting biotite gneiss with a northwest strike on the north shore of Shoe Bay, Rainy Lake, 162 TIte American Geologist. Marcli, 1891 afforded three specimens taken in the same seqnence as before. The first, from the contact, shows a pronounced porphyrite structure cor listing of a fine ground mass of phigioclase, augite iind magnetite in which are imbedded lath-shap'^d crystals of rather cloudy plagioclase of un earlier generation and polysomatic aggre- gates of pyroxene, which in many cases is partially altered to a serpentinous, greenish-yellov/ substance either on the periphery, having a fresh core, or in patches and shreds through the section. In the second specimen, taken at six feet from the contact, the structure is ophitic and in marked contrast to that of the last. The plagioclase is fresh, the augite is in scattered gi*ains and in polysomatic aggregates and is more or lesL' altered to hornblende. Quartz is present and is intergrown with the feldspar after the liianner of pegmatite. Magnetite occurs in skeletal forms and apatite in slender needles. In the specimens taken from tlie mid- dle of the dyke the general structure is granular rather than ophitic although the latter structure is observable. Augite is seen in occasional large pol3'somatic grains with a good deal of filmy or shreddy perimorphic hornblende and some chlorite. Hornblende also occurs in independent masses. Quartz is ver}' abundant and is nearly all intergrown with feldspar in pegmatitic structure. Magnetite and apatite are present, the former in irregular scat- tered grains and the latte'- in slender needles. A partial chemical examination of the sptcimens from this dyke gave the following figures for the percentage of silica and the specific gravity : Sp.g. Contact. 49.36 3.077 Middle. 51.04 3.007 Near the mouth of S\oe Bay, on the north side is another dyke similar to the last. It is about seventy feet wide. Thin sec- tions of three specimens taken from the same parts of the d3'ke as before, present the same general features as in the dyke last described. The rock at the contact is a porphyrite with the usual plagioclase crystals and augite aggregates imbedded in a fine- grained base. The latter is remarkable for the uniformly even distribution of the magnetite grains. The porphyritie augite has no crystal lographic boundaries and its alteration is for the most Petrographlcal Differentiation of certain dyhes. 1C3 t part marfjinr.l. No quartz was observed. At six feet the s^^ruot- ure is again ophitic. Quartz is present. IMagnotite is in large «ized, sparsely scattered grnins. A little pyrite also occurs. The structure of the middle part of the dyke is for the most part •granular hut with some idiomorphic plagioclase. The augite is generally fresh, but has associated with it hornblende and chlorite as alteiiition products. Quartz is abundant with uniform orienta- tion over wide, and in the section isolated, areas. The following are the figures for the silica percentage and specific gravity of the three specimens : Contact. Six feet from Middle, coiiiact. SiOj 48.85 47.93 49.28 3.088 3.079 i 3.016 Similar specimens were taken from a dyke on Risky Island, Bamy Lake, which is nearly in a line with the Stop Island dyke and the Shoe Baj^ dyke last described. The contact rock is as before distinctly a porphyrite similar to that represented in tig. I. The minerals are all fresh. Plagioclase in large crystals and augite in single and in polysomatic gnuns, are imbedded in the usual fine ground mass, which in this, and in all the dykes described, is ap- parently holocrystalline and micro-ophitic. At six feet fr^m the dyke wall the structure is ophitic but with some allotriornorphic plagioclase. The augite is mostly replaced by hornblende. Quartz is plentiful in pegmatitic develoi)ment. Magnetite or titanic iron is abundant but without any trace of leucoxene. In the middle of the dyke the ophitic structure is still ob.sorvable but most of the constituent minerals ai-e allotriomorphic. The augite is very largely replaced by hornblende. Quartz is observed to present the same ophitic relations to the idiomorphic plagioclase as does the {iugite, and it contains inclusions of apatite and microliter. Leucoxene with cleavage traces of titanic iron is abundant and in large grains. The following are the figures for the percentage of silica and specific gravity of these specimens: ^^ Contact. 49.77 3 100 Six feet from contact. 49.64 3.044 Middle. siOj Sp. g. .52.31 3.016 164 The American Geologist. March, 1891 On tlie south side of Rain}' river opposite Sec. 20, Tp. 5 S. ,. R. XXVIII of the Canadian township survey, a dyke was ob- served having a width of from 150 to 200 feet and cutting horn- blende schists with a north-northwest strilce. No specimen was here- obtained at tlie immediate contact, and in one a little removed from the contact the porphyritic structure was onl}' represented by- blebs of polysomatic augite imbedded in an ophitic base whichc approached in texture that of the specimens taken at four or six feet from the contact in other dykes. The ophitic structure pre- vailed in two other specimens, one taken at six feet from the con- tact and one from the middle. Quartz was observed in both of these but not in the first. The percentage of silica and specifie gravity of the first and third specimens is as follows : Near Contast. Middle. Sp. g. 49.82 3.221 50.10 3.068 Scries of specimens from several other dykes were also ex- amined, but the limit of space will not permit of further detailed descriptions. Generally, however, it may be said that the por- phyrite structure almost invariably characterizes the dyke rock at. the contact and that this rapidly grades into an ophitic structure which in turn appears to grade very gradually into the granular structure. The latter, it must be said, is developed to the entire exclusion of the ophitic structure only in a few of the cases ob- served. The increasing proportion of quartz toward the middle of the dykes is a very constant character. In one dj'ke, namely that on the south side of Rainy river opposite the town of Fort, Frances, well defined crystals of enstatite were observed in the rock at the contact as a porphyritic constituent while none of thia mineral was observed in other parts of the dyke. [From the Proceedings of the American Association for the Advancement OF Science, Vol. xxxviii.] Pbtrogiiaphical difficukntiation of certain dykes of the Rainy Lake region. By Dr. A. C. Lawson and F. T- Shutt, M. A., F. C. I., Ottawa. Out. [ABSTRACT.] Onc of the writers has described in a former paper certain diabase dykes of the Riv'.ny Lalie region. The present paper is the result of a more critical investigation of the same dylies, with others since discovered, having special reference to the petrographlcal differentiation of the dyke rock in passing from the contact walls to the centre of the dyke. The fact that dykes are very commonly fine grained at their margins and coarse grained in their middle parts is familiar to all geologists. On ex- amination of the dykes in question, it became apparent that this varia- tion in the pliysical appearance of the dykes is not simply one of texttire or degree of coarseness of the constituent minerals, but that it is rather the incidental concomitant of important structural, mineralogical and chemical variations which appear very constantly in the same way in dif- ferent dykes. These variations are chiefly as follows: Structural — the passage from the structure of a very fine textured diabase-porphyrite at the contact walls through the characteristic ophitic structure of diabase at a few feet from the contact to the granular structure of gabbro in the middle part of the dyke (Illustrative drawings were submitted.) Min- eralogical — the passage from a quartzless rock at the contact to a quartz- ose one towards the middle of the dyke. Chemical — the passage from a more basic rock near the contact to a more acid towards the middle. The results of complete or partial analyses by Mr. Shutt of series of speci- mens taken across a number of dykes were given in tabular form. The principal object of the paper is to adduce specific evidence that from a rock mass which is a geological unit of very limited extent, there may be taken specimens which under current methods of classification would receive diffe.ent names and be relegated to different classes. The fact that a series of specimens, in any given locality, differ frc n one an- other texturally, st/ucturally, mincralogicaliy and chemically, is no proof that they are not geologically the same rock crystallized from the same magma. The regular textural and structural differentiation of the dykes from wall to middle is inferred to have been caused by the difl'erent rate of cooling under constant pressure. The chemical differentiation Is prob- ably due to a seleccive crystallization of the more basic minerals in the earlier stages of solidification accompanied by the transference of acid residues from the sides to the middle by the agency of Included water. SALEM PRESS PUBLISHING AND PRINTING CO. I From the Proceedings op the American Association fob the Advancbmbny OF iiOIENCE, Vol. XXX Vm,] NOTK ON THK MAPPING OF TUB AUCH^AN NORTHWEST OF LaKB SUPE- RIOR. By Dr. Andrrw C. Lawson, Ottawa, Out. [abstract.] The writer exhibited a general geological map of the Archaean country between Red River Valley and Lake Superior, showing the results of re- cent investigations which he has been conducting for the Geological Sur- vey of Canada. The proof of a new geologically colored map of the Rainy Lake region, shortly to be issued by the survey, was also exhibited to illustrate the details of a portion of the general map. The relative distril)ution of tin; Upper and Lower Archaean, as displayed on the map, was shown to be peculiarly Interesting and instructive. The Lower Ar- chaean or Laurentian, consisting of various, more or less foluited granites and syenites, which have hitherto been regarded as the oldest rocks, was shown to occupy lai-ge, isolated boss-like areas which appear to be Ir- ruptive or intrusive through the schists. The general mapping of this portion of the country, where denudation has left the Upper and Lower Archaean In nearly equal proportions, strongly supports the view that the Laurontlan rocks are of later age than the schists of the Upper Archaean and were irrupted through them. The importance of careful mapping as an aid to the solving of the pro- found problems of Archaean geology was dwelt on, and other evidence which the writer has adduced elsewhere In support of the irrupt've nature of the Laurentian was referred to. Salem Press Publishing and Printing Co.