55o 



NATURE 



[Oct. 4, 1888 



series of crystalline schists are of sedimentary origin, and it is a 

 matter to be decided by detailed investigation which are to be 

 considered as sedimentary and which as eruptive or plutonic. 

 The results obtained by the author in the investigation of the 

 Saxon " Granulitgebirge" and some adjacent districts do not 

 therefore claim universal application. 



The tentative interpretations given by him were arrived at by 

 the close ob c ervation of the field-relations of the rocks in question 

 during a geological survey extending over several years ; and it 

 ■can now only be a question in how far the interpretation, which 

 has been recognized with certainty as correct fcr a series of 

 phenomena, can be applied to other phenomena intimately 

 related to them. The author admits that here and there he has 

 gone somewhat too far in his tentative interpretation. It was 

 scarcely possible, in so difficult a question as the '* granulite 

 question, ' which to-day has not yet reached its final limits, to 

 go just so far that later experience should find nothing to modify. 

 But the description of the author's work by J. Roth (in a paper 

 on "Zobtenite," read before the Berlin Akademie der Wissen- 

 ' schaften on June 23 of last year) as " a marvellous agglomeration 

 of the most daring hypotheses " is scarcely justifiable. 



In Prof. Lehmann's investigations on the crystalline schists it 

 has, for the first time, been shown in the greatest detail that their 

 present condition cannot be original, but must be one that has 

 been influenced by the dynamic processes accompanying mountain- 

 building. He is far from maintaining, however, that similar 

 observations had not already been made ; and he readily 

 acknowledges that eminent investigators of the crystalline 

 schists, such as Kjerulf and Michel- Levy, had, at a much 

 earlier period, made such observations. What is new is the 

 mode and method in which the author utilizes his observations. 

 Researches of this kind were sunk into oblivion : the theory of 

 the sedimentary origin of the crystalline schists had become the 

 ruling dogma ; and the Eozoon canadense had also made its 

 appearance in Europe. 



Roth, in the paper referred to, maintains his old position, 

 according to which the crystalline schists, including the phyllites, 

 are plutonic and unaltered formations. 



The evidence advanced by him to prove that the stratiform 

 gabbros, which he terms zobtenite, cannot be numbered with 

 the eruptive rocks is insufficient. The occasional observation of 

 conformable relations with other crystalline schists is inadequate. 

 This does not, however, hinder Roth from regarding it as proved 

 that the Zobten rock cannot be eruptive. The isolated patches 

 of the old rocks that crop out in Silesia are unfortunately 

 extremely confused. The stratigraphical relations of these rocks, 

 which are very highly metamorphosed, cannot be utilized to 

 support either view, and no hope is to be entertained of more 

 favourable exposures in the future. 



Prof. Lehmann's views on the Saxon granulites have, in the 

 main, been confirmed by the before-mentioned work of Herr E. 

 Danzig. This work again shows how confused are the field - 

 relations in the granulite-district, and that few exposures permit 

 of an indisputable solution. 



In the northern half of the Saxon district the granulite assumes 

 a granular structure, and acquires a marked similarity «o some 

 " bedded" granites. These points have received especial atten- 

 tion from Herr E. Danzig. He comes to the conclusion that in 

 many places no sharp line can be drawn between granulite and 

 granite ; further, that rocks, which belong undoubtedly to the 

 granulites, present, like the granitic gneisses occurring in the 

 granulite-complex and interbedded with mica-schists, thecharacter 

 of eruptive masses. They contain included fragments, and im- 

 pregnate these as well as their immediate neighbourhood. The 

 supposition formulated by Prof. Lehmann at the close of his 

 researches in this district is thus confirmed — namely, that the 

 Saxon granulite is a granite massif, which has been influenced in 

 structure and composition by dynamic metamorphism. 



This confirmation of his work induces the author to explain 

 why he cannot accept the views advocated by E. Reyer in his 

 newly-published work on "Theoretical Geology." Reyer holds 

 the Saxon granulite- waj«/ for "a mass, of eruptive granite 

 (Massencrguss), mantled over by ' tuffogenic ' sediments 

 {granulite), through which granite dykes a*-e extruded from 

 the central mass ; while granite sheets (Flankencrgiisse) are 

 intercalated between its beds." Reyer might have gathered 

 from the author's work that the Saxon granulites are, in the 

 main, by no means highly metamorphosed : on the contrary, 

 they deviate very little, in part not at all, from the original 

 structure of eruptive granite rocks. 



But apart from this, and without dwelling on the fact that we 

 know absolutely nothing of the rocks underlying the Saxon 

 granulites, the supposition that the alternation of mica-schists 

 with granulite or granitic gneisses has been produced by an 

 accumulation of successive lateral eruptions {Flankenergiisse) and 

 precipitated sediments, cannot hold good. 



The theoretical considerations of Reyer, the utility of which 

 is gladly recognized by the author, and which in many cases 

 can be supported by direct observation, must not be allowed to 

 prejudice our judgment. The actual facts must first be estab- 

 lished, and in so doing we do not encounter the streaky and 

 platy structures which characterize the direction of movement in 

 magmas. We see, in truth, something quite different. The 

 "bedded" granite presents no zones of consolidation that 

 follow closely the surrounding slates ; we see rather an extra- 

 ordinarily uniform mass of granite at first traversing, in a dyke- 

 like manner, the slates, but afterwards insinuating itself between 

 them, in both cases enclosing fragments of the t raver.- ed rock. 

 Where the granite was intruded as a dyke these fragments lie 

 without order, but where it forms a sheet the flat pieces are, 

 almost without exception, arranged parallel to the walls of the 

 dyke. We are accustomed to regard granite, occurring as a dyke, 

 as younger than the rock in which was formed the crack along 

 which the molten rock ascended, without wishing to deny that 

 it has existed, from the very beginning, deeply hidden in the 

 bowels of the earth, and is therefore, in reality, older than the 

 slates it traverses. But it has become customary to observe 

 the convention ; indeed, it is necessary to do so if we do not 

 wish to be involved in universal chaos. 



For the "bedded" granite it is no simple matter to prove 

 that it is younger than its hanging wall. Attentive examinaiion 

 shows that the apparently conformable boundary has no such 

 very conformable course ; that, further, the apparently sedi- 

 mentary beds are sometimes distinctly detached, and turn out to 

 be loose masses ; finally, a whole series of detailed phenomena 

 show that wherever there have been dislocations, the granite has 

 followed the opening and has impregnated the slates. How far 

 such an impregnation can be assumed to have taken place is a 

 matter for personal experience. 



In the granite dykes the inclusions and the boundary surfaces 

 of the slates present exactly the same phenomena ; only in this 

 case the fragments do not all present a parallel arrangement. 



One would be driven to deny the possibility of strata or slate- 

 masses being split parallel to their stratification or their bedding, 

 if we were to deny that the "bedded" granites do not as much 

 constitute a case of intrusion along cracks as do the obliquely- 

 running granite dykes. Why should there not be, among such a 

 number of granite dykes that run unconformably, some that have 

 been formed by the in-filling of cracks (of seldom more than 

 400 metres width) that follow the divisional planes of strati- 

 fication or cleavage ? It is not to be supposed that these were 

 cavities, the wide sweeping arches of which were supported by 

 the rigidity of the lateral rock-masses : as fast as the slates were 

 separated the granite forced in its way, and filled up the crack 

 as soon as it was developed. 



This separation along parallel divisional planes and intimate 

 impregnation with eruptive material, which can be followed in 

 the minutest details with the greatest clearness, arouse the ques- 

 tion as to whether the same phenomena have assumed greater 

 dimensions— dimensions that would still be trivial in comparison 

 with the masses erupted. The author has described several 

 exposures in the Saxon granulite district that render any other 

 interpretation impossible. 



Kjerulf, Michel- Levy, and others have described very similar 

 relations among eruptive masses. Michel- Levy has quite recently 

 given expression to his opinions in a" Note sur l'origine des 

 terrains cristallins primitifs," and in a " Note sur les roches 

 eruptives et cristallines des montagnes du Lyonnais." His 

 statement to the effect that the audior and a portion of the 

 German school assume a development of heat by the plication of 

 the earth's crust is, so far as the author is concerned, incorrect. 

 On the contrary, he has shown that a c nvn-sion of motion 

 into heat has left ro visible traces. He is quite at one with 

 the French investigator as to the origin of the heat in the earth's J 

 crust. 



The chief requisite in the discussion of the crystalline schists, | 

 is never to leave the solid ground of facts, and to pay particular 

 attention to the collecting of these. If the statements of some 

 authors are examined, it must awake astonishment to see with 

 what positiveness statements are made, which, although of the j 





