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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%<>« 2.H) .94 2.02 l-usson ig. 2.05 2.53 2.25 {)