J3L 
 
 
 IMAGE EVALUATION 
 TEST TARGET (MT-3) 
 
 4:o 
 
 
 1.0 
 
 if I.I 
 
 l^|2£ 112.5 
 
 ^ m 
 
 ^ m 
 
 ^ 14.0 
 
 L25 i 1.4 
 
 1.6 
 
 v) 
 
 cf-J 
 
 c?2 
 
 f> > 
 
 
 
 o 
 
 / 
 
 W 
 
 .--.-i..:i..M^.-.^ 
 
 
 Hiotographic 
 
 Sdences 
 Corporation 
 
 23 WEST MAIN STitEET 
 
 Wr'tSTER.N.Y. 14580 
 
 (716)S73-4S03 
 
CIHM/ICMH 
 
 Microfiche 
 
 Series. 
 
 CIHIVI/ICMH 
 Collection de 
 microfiches. 
 
 Canadian institute for Historical Microreproductions / Institut Canadian de microreproductions historiques 
 
Technical and Bibliographic Notes/Notes techniques et bibliographiques 
 
 The Institute has attempted to obtain the best 
 original copy available for filming. Features of this 
 copy which may be bibliographically unique, 
 which may alter any of the images in the 
 reproduction, or which may significantly change 
 the usual method of filming, are checked below. 
 
 D 
 
 D 
 
 a 
 
 D 
 
 Coloured covers/ 
 Couverture de couleur 
 
 □ Covers damaged/ 
 Couverture endommagde 
 
 □ Covers restored and/or laminated/ 
 Couverture restaur^e et/ou pellisul^e 
 
 Cover title missing/ 
 
 Le titre de couverture manque 
 
 I I Coloured maps/ 
 
 D 
 
 Cartes g6ograr«hiques en coutaur 
 
 Coloured ink (i.e. other than blue or black)/ 
 Encre de couleur (i.e. autre que bleue ou noire) 
 
 Coloured plates and/or illustrations/ 
 Planches et/ou illustrations en couleur 
 
 Bound with other material/ 
 Relid avbc d'aufes documents 
 
 Tight binding may cause shadows or distortion 
 along interior margin/ 
 
 La reliure serr6e peut causer de I'ombre ou de la 
 distortion le long de la marge int^rieure 
 
 Blank leaves added during restoration may 
 appear within the text. Whenever possible, these 
 have been omitted from filming/ 
 I! se peut que certaines pages blanches ajoutdes 
 lors d'une restauration apparaissent dans le texte, 
 mais, lorr/ijue cela ^tait possible, ces pages n'ont 
 pas 6t6 filmies. 
 
 Additional comments:/ 
 Commentaires suppl^mentaires; 
 
 L'lnstitut a microfilmd le meilleur exemplaire 
 qu'il lui a 6t6 possible de se procurer. Les details 
 de cet exemplaire qui sont peut-dtre uniques du 
 point de vue bibliographique, qui peuvent modifier 
 une image reproduite, ou qui peuvent exiger une 
 modification dans la mdthode normale de filmage 
 sont indiqu6s ci-dessous. 
 
 □ Coloured pages/ 
 Pages de couleur 
 
 □ Pages damaged/ 
 Pages endommagies 
 
 r~V Pages restored and/or lamin; ted/ 
 LlJ Pages restaur4es et/ou pelliculdes 
 
 ri^ Pages discoloured, stained or foxed/ 
 I I Pages ddcolor6es, tachet^es ou piqu^es 
 
 I I Pages detached/ 
 
 n 
 
 Pages ddtach^es 
 
 Showthrough/ 
 Transparence 
 
 Quality of prir 
 
 Quality in6gale de {'impression 
 
 Includes supplementary materii 
 Comprend du materiel supplementaire 
 
 I T Showthrough/ 
 
 ["~"| Quality of print varies/ 
 
 I I Includes supplementary material/ 
 
 Only edition available/ 
 Seule Edition disponible 
 
 Pages wholly or partially obscured by errata 
 slips, tissues, etc., have been refilmad to 
 ensure the best possiblo image/ 
 Les pages totalement ou partieilement 
 obscurcies par un feuillet d'errata, une pelure, 
 etc., ont 6t6 filmdes d nouveau de fapon it 
 obtenir la meilieiire image possible. 
 
 This item is filmed at the reduction ratio checked below/ 
 
 Ce document est film4 au taux de reduction indiqud ci-dessous. 
 
 10X 
 
 
 
 
 14X 
 
 
 
 
 1BX 
 
 
 
 
 22X 
 
 
 
 
 26X 
 
 
 
 
 30X 
 
 
 
 
 
 
 
 
 
 
 
 
 
 J 
 
 
 
 
 
 
 
 
 
 
 
 
 
 12X 
 
 ItfX 
 
 20X 
 
 24X 
 
 28X 
 
 32X 
 
The copy filmed here has been reproduced thanks 
 to the generosity of: 
 
 Library, 
 
 Geological Survey of Canada 
 
 L'exemplaire film6 fut reproduit grAce i la 
 
 g6n6rosit6 de: 
 
 Bibliothique, 
 
 Commission G6ologique du Canada 
 
 Tne images appearing here are the best quality 
 possible considering the condition and legibility 
 of the original copy and in keeping with tha 
 filming contract specifications. 
 
 Original copies in printed paper covers are filmed 
 beginning with the front cover and ending on 
 the last page with a printed or illustrated impres- 
 sion, or the back cover when appropriate. All 
 other original copies are fil.ned beginning on the 
 first page with a printed or illustrated impres- 
 sion, and ending on the last page with a printed 
 or illustrated impression. 
 
 The last recorded frame on each microfiche 
 shall contain the symbol — ♦► (meaning "CON- 
 TINUED"), or the symbol V (meaning "END"), 
 whichever applies. 
 
 Maps, plates, charts, etc.. may be filmed at 
 different reduction ratios. Those too large to be 
 ertirsly included in one exposure are filmed 
 begi ming In the upper left hand corner, left to 
 right and top to bcttor>->. as many frames as 
 required. The fo'Sowing diagrams illustrate the 
 method: 
 
 Les images suivantes ont 6t6 reproduites avec le 
 plus grand soin, compte tenu de la condition et 
 de la nettet6 de l'exemplaire film6. et en 
 conformity avec les conditions du contrat de 
 filmago. 
 
 Les exemplaires originaux dont la couverture en 
 papier 9St imprimis sont fiimis en commengant 
 par ie premier piat et en terminant soit par la 
 dernidre page qui comporte une empreinte 
 d'impression ou d'illus»ration, soit par le second 
 plat, salon le cas. 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.