

524 



NATURE 



[Sept. 27, 1888 



of a decimetre diameter, composed of macropegmatite, enveloping 

 a porphyritic Carlsbad twin of potash-feldspar (orthoclase or 

 microcline), that occur in the granite of the Riesengebirge, 

 repeat, on the large scale, the microscopic characters of the 

 micropegmatite of certain quartz- and granite porphyries (the 

 granophyre of Rosenbusch). All these phenomena compel the 

 assumption that at least a part of the pegmatites are of indubit- 

 ably eruptive origin, and arouse in us the question whether this 

 structure is not to be brought into connection with the origin of 

 the gneisses. 



(5) What are the differences between the primary structures 

 {due to consolidation) of the plutonic and volcanic rocks and the 

 structures of (a) the crystalline sediments, (b) the metamorphic 

 rocks in contact with granite, (c) the crystalline schists? 



(6) What reliable characters have we, to distinguish crystalline 

 grains developed in situ from clastic grains, in cases where they 

 occur, side by side, in one and the same rock ? 



The answer to this question has already frequently been 

 attempted, among others in the most praiseworthy manner by 

 A. Wichmann. It requires, however, a fresh solution based 

 on the latest experiences. The safest test of the authigenic, 

 non-clastic nature of a grain is doubtless the presence in it of 

 enclosures of minerals that are also present in the rock as 

 authigenic constituents. External form and internal molecular 

 relations, in consequence of pressure-phenomena, can, how- 

 ever, be very misleading. Hard minerals, especially, occur 

 in clastic sand in very sharp crystals (quartz, tourmaline, 

 zircon, &c). 



(7) Are the views of those authors justifiable, who conceive 

 certain gneisses or porphyroid crystalline schists to have been 

 produced by the injection of a granitic magma, in discontiuuo, 

 between the schists {Schiefer) ? 



(8) If the views expressed in the preceding question are justi- 

 fiable, how are the gneisses and porphyroids, produced by the 

 addition of granite in diicontinuo to slaty sediments, to be 

 distinguished (a) from true eruptive granite or its porphyritic 

 modification, both having, under the influence of pressure, under- 

 gone a "phyllitic" modification ; (b) from slaty sediments in 

 which aggregates or crystals of silicates have been deposited 

 from water (quartz and feldspar)? 



(9) What differences can be established in mineral composition 

 and structure between a true eruptive granite and an indubitably 

 stratified (not simply jointed or cleaved) so-called " Lagergranil" 

 or granite-gneiss ? 



An amalgamation of eruptive g>-anite with the mineral 

 aggregates of the rocks in contact has, according to my experience 

 taken place in some cases ; but I have not yet observed an 

 undoubted discontinuity in such granitic material. It is much to 

 be desired that the French geologists (for instance, Michel- Levy 

 and Charles Barrois), who defend the views formulated in Ques- 

 tions 7 and 8, would enlighten us by good drawings of macro- or 

 microscopic sections, as to how far in this difficult question an 

 incontestable separation of injected iruptive grani'e from meta- 

 morphic gneiss is possible. This would, without doubt, facilitate 

 the solution of Question 9. Unanimity on this point will scarcely 

 be obtained without a careful structural diagnosis, which, of 

 course, must be supported by serviceable material, self-collected 

 in the field. 



(10) Are there any absolute material and structural differences 

 between metamorphic rocks of the granite contact-zone (horn- 

 stones, corneenne, &c, cp. Question 1) and rocks affected by 

 regional or dynamic {Dislocations-) metamorphism ? or are such 

 differences only relative, and what are they? 



The exact solution of this question requires, above all, the 

 assumption that only such occurrences shall be submitted to con- 

 sideration that are unmistakably connected with visible eruptive 

 rocks. It should also not be forgotten that rocks which have 

 originally undergone contact-metamorphism have, in some cases, 

 stibsequently lost their peculiar characteristics in consequence of 

 the influence of regional metamorphism. With this qualification 

 I am personally inclined to concede only a relative and not 

 absolute differences. I am guided in this, not only by my 

 experience in the Hartz, which has made me acquainted with 

 the remai kable variation of the metamorphic rocks in contact 

 with granite, according as they occur just outside the contact- 

 zone or in its outer, middle, or inner division ; or again 

 according as they belong to the impenetrated but eroded mantle 

 of the eruptive cores, or to masses, of greater or smaller extent, 

 that have sunk deep in between the eruptive masses and have 

 been covered up by them. The rocks occurring thus differently 



vary between a phyllitic clay-slate and gneiss, while the main 

 mass of the slate- and grauwacke-hornstones present little 

 resemblance to the crystalline schists. In the classic region of 

 the Erzgebirge, however, there occur, according to the careful 

 investigation of our Saxon colleagues, compact hornstone-likc 

 or even conglomeratic greyivacke gneisses (the mica-trap of older 

 writers) that present this analogy in a complete degree. 

 The same analogy is presented by Gosselet's Lower Devonian 

 " corneile" (to be distinguished from cornienne, the product 

 of contact-metamorphism) from the regionally metamorphic 

 Ardennes of Belgium. Again, the Lcnver Devonian fossiliferous 

 tediments of the Ardennes, containing garnets, hornblende, and 

 graphite, that are so well known through A. Renard's admirable 

 descriptions and drawings, remind one of hornstone, although 

 no contact with eruptive rocks has been observed affecting either 

 them or the Cambrian garnetiferous " Wetzschiefer" of Vielsalme. 

 The association of such hornstone-like rocks with those of the 

 usual phyllitic type of regional metamorphism recalls the 

 occurrence of lime silicate-hornstones in the outermost zone 

 (beyond the zone of the " Knotenschiefer " around the granite 

 of the Rammberg. Whatever explanation of these phenomena 

 may be given — Gosselet is decidedly in favour of dynamic 

 metamorphism as opposed to a latent contact-metamorphism — 

 at least this is evident, that important contributions to the 

 question, here formulated, can be furnished by the Ardennes. 



ON THE CLASSIFICA TION OF THE 

 CR YS TA LLINE SCHIS TS} 



"T^HE most important constituent of the earth's crust — the 

 -*■ crystalline schists — has remained, with respect to their 

 field-relations and their origin, the most shrouded in dark- 

 ness. The difficulties thaf bar the way are quite excep- 

 tional. We have frequently to deal with rocks that have 

 undergone subsequent alteration, without being able to determine 

 their original constitution, and without being able to explain the 

 nature of the change. We have, as it were, to deal with an 

 equation with two unknowns — we cannot solve it. 



At the present time we meet with a number of attempts to 

 classify the crystalline schists, mainly according to petrological 

 characters, in stratigraphical groups. I regard these attempts 

 as premature, for this reason : microscopists are unfortunately 

 very behindhand in the exact investigation of the crystalline 

 schists, and of the half-clastic, half-crystalline sediments. The 

 purpose of these lines is to direct attention to another difficulty 

 which has not yet received sufficient consideration, but which 

 bars the way to every attempt of that kind — namely, the 

 mechanical metamorphism during mountain-formation. 



That, by the plication of the Alps, the constitution of the rocks 

 has been completely changed, is most directly proved by an ex- 

 amination of the sedimentary rocks ; becau-e the latter can be 

 also studied in an unaltered condition in adjacent localities. The 

 commonest changes met with here in connection with folding 

 are : — 



Deformation of fossils, pebbles, or crystals (compression in 

 one direction, extension in another) 

 Cleavage ( Transversalschiefa ting). 

 Cleavage with linear extension. 

 Puckering. 



Internal formation of breccias and cementing of the same by 

 secretions. 



Internal formation of innumerable slickensides, so as to 

 change the whole structure. 



Scaly structure, produced by the compression of oolitic 

 structure. 



Alteration of hematite and limonite into magnetite, in 

 connection with cleavage. 

 Marmorosis of the limestones. 



Formation of confusedly "kneaded" structures {KnetsU-uc- 

 turen). 



Development of new minerals (garnet, staurolite, mica) in 

 places that have undergone crushing. 



Now, sedimentary rocks, metamorphosed in the above way, 

 are frequently found in extremely narrow synclinal zones, nipped 

 in between rocks belonging to the crystalline schists. The 



1 "Zur ^Classification der krystallinischen Schiefer," von Prof. Dr. Albert 

 Heim. "Etudes -ur les Schistes Cristallins." Published by the Inter- 

 national Geological Congress in London, 1888. (Translated from the German 

 by Dr. F. H. Hatch.) 



