5 22 



NA TURE 



[Sept. 27, 1888 



The type of rocks characterized by orthoclase, appearing in the 

 fundamental granite and the granitoid gneisses of the Lauren- 

 tian, is again found in the quartziferous porphyries of the 

 Arvonian, in the Montalban gneisses, and, though less distinctly, 

 in the feldspathic rocks of the Taconian. The non-magnesian 

 micas, rare in the fundamental granite and the Laurentian 

 gneisses, appear abundantly in the Montalban gneisses and mica- 

 schists, as well as in the lustrous schists which are found in the 

 Huronian and the Taconian, and which predominate in the 

 latter. It is further to be remarked that the simple silicates of 

 alumina, such as andalusite, cyanite, fibrolite, and pyrophyllite, 

 as yet unknown in the more ancient terrane=, are abundant in 

 the Montalban, and are also found in the Taconian. At the 

 same time, crystalline limestones, oxides of iron, and calcareous 

 and magne=ian silicates, are met with in every terrane above the 

 fundamental granite. 



The chemical and mineralogical differences between these 

 various terranes are more remarkable than the resemblances, a 

 fact which, however, has not prevented some observers from 

 confounding the younger with the older gneisses. Again, the 

 resemblances between the Huronian and Taconian terranes led 

 the late Prof. Kerr, in North Carolina to refer the latter terrane 

 to the Huronian. Moreover, in the vicinity of the Lakes Superior 

 and Huron, where we find alike Laurentian, Nr.rian, Huronian, 

 Montalban, .and Taconian, the outcrops of this last were con- 

 founded with the Huronian by Murray and by other observers. 

 In 1873, however, the author, distinguishing between the two, 

 gave to the Taconian in this region the provisional name of the 

 Animikie series. It was not until later that he recognized the 

 fact that this series, which is here found in certain localities 

 resting unconformably upon the Huronian, is no other than the 

 Taconian. Emmons, on the contrary, who had long known the 

 existence in this region of what he called the Lower Taconic, 

 believed that the terrane to which the author gave, in 1855, the 

 name of Huronian, was identical with this same Lower Taconic 

 or Taconian. The differences between these two terranes in the 

 basin of Lake Superior, first noted by Logan and later by the 

 author, are clearly brought out by the recent studies of 

 Rominger. 



Upon all these different terranes, including the Taconian, 

 there rests in discordant stratification in this region a vast series 

 of sandstones and conglomerates, with contemporary basic plu- 

 tonic rocks, the whole remarkable by the presence of metallic 

 copper. This series, which had been alternately confounded 

 with the Huronian and the Taconian on the one hand, and with 

 the trilobitic sandstones of the Cambrian on the other, was for the 

 first time separated by the author in 1873, under the name of the 

 Keweenaw group, a term changed by him in 1876 to that of the 

 Keweenian terrane. It still remains to be decided whether this 

 series, upon which rest unconformably these same trilobitic sand- 

 stones, should form a part of the Cambrian, or should constitute 

 a distinct terrane between the Taconian and the Cambrian. 



§ 11. In submitting to his colleagues of the International Geo- 

 logical Congres? this summary of his conclusions, based on over 

 forty years of study, the author takes the liberty to state that the 

 notions here advanced as to the origin, the chemical and minera- 

 logical history, the subdivision, and the nomenclature of crystal- 

 line rocks, are for the most part the generalizations of a single 

 observer. He now offers them as a first attempt at a classifica- 

 tion of the indigenous rocks, and at the same time as an exposi- 

 tion of his crenitic hypothesis, and of the mineralogical evolution of 

 the globe, which he conceives to have determined the succession 

 and the chemical nature of the masses which he has named 

 crenitic, as well as those of plutonic masses. He feels at the 

 same time that his work is far from complete, and that to others 

 must now be left the task of correcting and finishing it. 



As a large part of these results, so far as regards geognostic 

 classification, appeared for the first time in the Reports of the 

 Geological Survey of Canada, the author may be permitted to 

 say, in closing, that the first publications made by that Geo- 

 logical Survey on the crystalline rocks of Canada — that is to say, 

 the reports of progress for the years 1845 and 1846, were pre- 

 pared by him, and published in 1847, from the notes and the 

 collections made by Logan and by Murray in the two years 

 previous. Moreover, all the statements relating to the minera- 

 logy, the lithology, or the chemical composition of the rocks 

 of Canada, which are found in the official reports from 1847 to 

 1872, when the author resigned his position as a member of the 

 Geological Survey of Canada, were written bv him or under his 

 personal direction. . T. Sterry Hunt. 



SOME QUESTIONS CONNECTED WITH THE 

 PROBLEM PRESENTED BY THE CRYS- 

 TALLINE SCHISTS, TOGETHER WITH 

 CONTRIBUTIONS TO THEIR SOLUTION 

 FROM THE PALAEOZOIC FORMA TIONS. 1 



'T" , HE question of the "crystalline schists" still presents so 

 many unsolved difficulties, and the views of contempo- 

 raneous fellow-workers diverge herein so widely, that an 

 attempt at unanimous agreement on the points at issue must at 

 present be regarded as premature. This assuredly does not 

 prevent our taking counsel together, interchanging observations, 

 and endeavouring to gain solid ground, whence a future solution 

 can be aimed at. Each geologist will approach such a con- 

 sultation in a way differing in accordance with his own 

 experience. 



I can only contribute experience gained by the study of the 

 metamorphic crystalline schists, belonging to the PaLcozoic 

 formations, that have been proved to have resulted) from the 

 action of contact or dynamic metamorphism on eruptive or 

 stratified rocks, the latter including the tuffs. The direct appli- 

 cation of this experience to all Arcrnean crystalline schists 

 appears to me premature— i.e. rather a thema probandum than 

 probatum. Doubtless there are cases — as, for instance, in the 

 so-called flasergabbros or zobtenites, which, apparently, must be 

 regarded as quite analogous to the alteration of the diabases in 

 the Pala'ozoic formations. Indeed, the same essential features 

 which Lehmann has described in the development of the Saxon 

 "flasergabbros" have been demonstrated by Teall in the Lizard 

 gabbros, G. H. Williams in the Baltimore gabbros, and Hans 

 H. Reusch in Norway. But Hans H. Reusch also mentions 

 bedded gabbros^ as well as eruptive flasergabbros, differing thus 

 from Lehmann ; while Credner and Roth appear by no means 

 willing to concede all that is contained in Lehmann's book. 

 This fundamental difference must, however, be noticed : Lehmann 

 holds the Archaean schists half for metamorphosed sediments, 

 half for interbedded or injected eruptive rocks ; and although I 

 cannot agree with or follow Lehmann in every detail (and, above 

 all, lay more stress upon the altered tuffs), still on the whole I 

 can but support him in this view. Roth, on the other hand, 

 holds all the Archaean crystalline schists— limestones, quartzite, 

 gneiss, mica-schist, amphibolite, &c. — for schistose, plutonic 

 (only in form not eruptive) rocks {Erstarrungskrusle) ; finally, 

 Credner holds the majority of the crystalline schists, including 

 granite-gneiss and flaser-gabbro, for the normal stratified sedi- 

 ments of a primeval ocean, their crystalline nature being essentially 

 not due to metamorphism. 



I have dwelt thus at length on this point in order to demon- 

 strate that there exist numerous controversies even on those 

 questions that admit of solution by reason of the most undoubted 

 pseiidomorphic changes (hornblende after diallage, hypersthene, 

 augite ; zoisite, epidote, actinolite, quartz, albite after lime-soda 

 feldspar), and by reason of the presence of the 01 iginal eruptive 

 structure. 



My stand -point is identical with that expressed by Carl 

 Friedrich Naumann in the following words : My task above all 

 else is to study the metamorphism, with respect both to substance 

 and to structure, ofthefossiliferous sediments and the eruptive rocks, 

 together with the tuffs intercalated therein. Much has already 

 been done, especially with respect to contact-metamorphism, 

 which is more sharply defined than regional or dynamic meta- 

 morphism. There remains, however, much to answer, es- 

 pecially as the primary structures of original schistose eruptive 

 rocks and the structure and substance of certain very common 

 sedimentary rocks (as, for instance, the greywackes, the so- 

 called gieywacke-schists, or the majority of the tuffs) are 

 still too little known to afford a firm basis for the study of 

 metamorphic processes. 



Still the detailed solution of the following question would be 

 of no little value for the study of the Archaean schists : — 



(1) What material agreement or difference exists between the 



1 " Einige Fragen zur Losung des Problems der krystallinischen Schiefer, 

 nebst Beitiagen zu deren Beantwortung aus dem Palaoz^icum," von Prof. 

 Dr. R. A. Lossen. " Etudes sur les Schistes cristallins," 1888. Published 

 by the International Geological Congress in London, 1888. (Translated from 

 the German by Dr. F. H. Hatch.) 



2 Giving a s mewhat wide meaning to the word "gabbro"; he now says, 

 "dioritic rock," "altered gabbro and diabase." In the Hartz the interest-. 

 ing gabbro-district of Hartzburg presents, among numerous other varieties, 

 some which show layers alternately richer in plagioclase and diallage (bronzite) 

 or present _/?rt.tcr-structure with biotite. and possess thus a bedded-like but 

 1 ot a true I edited parallel structure. These r^cks are true erupiive gabbros. 



