Sept. 27, 1888] 



NATURE 



525 



Alpine zones, which consist mainly of crystalline schists, are 

 termed antral massifs. Such intercalations of mechanically 

 metamorphosed sediments with the crystalline schists are very 

 frequently to be observed at the ends of the strike of the central 

 massifs, and between the central massifs ; they are not rare even 

 in the interior of the central massifs. The crystalline schists 

 and metamorphosed sediments not only present the fame strati- 

 graphical position, but also similar characters in other respects. 

 The cleavage of the sedimentary rocks may be continued in the 

 same direction into the crystalline schists ; and similar contor- 

 tions may traverse both : in the latter, as in the former, a marked 

 linear extension in the same or but slightly deviating direction 

 may be present : calcareous patches in the crystalline schists are 

 crystalline and granular, and contain layers of mica-scales which 

 have undergone extension, precisely as in the neighbouring 

 Jurassic limestones, &c, &c. From these facts we see that 

 in these crystalline schists we have not to deal with rocks of 

 original constitution, but that both these rocks and the 

 sediments have undergone similar mechanical metamorphism. 

 The only difficulty in dealing with the schists is contained in the 

 fact that we are never in a position to describe the original ap- 

 pearance of the rock before it underwent the mechanical 

 metamorphism. 



Now it is in the crystalline schists that the plications of the 

 earth's crust are most potently developed. The isoclinal and 

 fan-shaped folds, the wedging and "kneading together" at the 

 contact with the sediments— in short, all these high forms of dis- 

 location, which are the earliest to modify the inner structure of 

 rocks, are to be found in the crystalline zones of the Alps. They 

 are most highly developed in the northern series of the central 

 massifs (Mont Blanc, Aiguille Range, Finsteraar-w^j-?/; 

 GotthaxA- massif, SWvTette-massif, &c). 



At first sight it appears as if the crystalline schists and the 

 true sediments, in the Alps, were separated by a constant un- 

 conformity ; but frequently even recent sediments are found 

 folded in, parallel with the crystalline schists. Again the 

 sediments often take the position of a central massif; indeed, it 

 seems as if a great part of several of the central massifs consisted 

 of Palaeozoic sediments. On the other hand, in the southern 

 central massi/s of the Central Alps, we see the crystalline schists 

 lying in all respects like the sediments. 



Those who have worked in these parts of the Alps will 

 have remarked how often the mechanical crushing under- 

 gone by the rocks obliterates the limits of stratigraphical and 

 petrographical characters, and how many rocks have become 

 confused thereby in their development {Ausbildungsweise). , 

 Such changes can sometimes be directly proved to be the result 

 of local crushing ; sometimes, however, they are regional, and 

 then passages into the unaltered rock are difficult to trace. All 

 degrees of change by earth-movements are to be found, from a 

 slight alteration of the structure up to complete metamorphism. 

 In hundreds of places one does not know whether one has to 

 deal with the residual traces of original bedding or with 

 a cleavage ( Transversalschieferung, Quetschungsschieferung) 

 that has completely obliterated the original structures. In 

 many cases it is impossible to distinguish between a schistose 

 structure (Schieferung), superinduced by earth-movements, and 

 one that is original. Schistose structures which cross one 

 another are by no means rare. Whether the more pronounced 

 or the less definite one is then the original is often not to be 

 decided. Even an exact microscopical examination will often 

 not suffice to distinguish between structures resulting from crush- 

 ing and lateral deformation, and the fluxion-structure of an 

 eruptive rock. It is certain that a structural modification by 

 •earth-movements has everywhere taken place where linear ex- 

 tension abounds. The latter is never original. In such 

 crystalline schists with linear-parallel structure there are often 

 elongated, ragged mica-scales. The linear extension can go 

 as far as the development of rod-like separation {stenglige 

 Absonderung). 



Are there any rocks left in the central massifs of the Alps 

 which have undergone no change in structure during the 

 orogenetic processes? 



The metamorphism can penetrate still deeper. 



Enormous zones, for instance, in the interior of the Finsteraar- 

 massif that were formerly held to be true crystalline schists, 

 prove to be originally clastic rocks of the Carboniferous period 

 that have been squeezed into schists, and pervaded by 

 secondary mica. Conglomeratic rocks of the Verrucano group, 

 and clay-slates, nipped into the central massif, have become 



crystalline, schistose, and even gneissose. They can scarcely be 

 distinguished, in the field and in the hand-specimen, from 

 crushed gneisses pervaded by sericite. Granites can be proved, 

 locally and perhaps also regionally, to have been compressed 

 into gneisses. Gneisses, having a different position relatively 

 to the pressure, have locally become granitoid. Massive 

 eruptive felsite-porphyries have become felsite-schists. Mica- 

 schists have been dragged out; their quartz grains ground 

 down ; and the whole converted into a rock that one would be 

 inclined to describe as a sandy clay-slate. Even l.iassic slates 

 with fossils have been converted into garnetiferous mica-schists, 

 staurolite-schists, &c. The boundary between the old crystalline 

 schists and real sediments in the Alps has, by such processes 

 of dynamic metamorphism, been obliterated, and the proper 

 character of the rock so altered as to render recognition impos- 

 sible. When we see, in true sediments, new minerals developed 

 by the progress of the mechanical metamorphism (magnetite in 

 the crushed Oolitic ironstone of the Winctgalle, garnet in the 

 Beleuonite-slates of Scopi), the question arises, for the crystalline 

 schists of this and neighbouring regions — Which minerals are 

 original, and which have been produced subsequently, by 

 orogenetic processes ? 



We arrive at this conclusion : — The constitution of the crystallim 

 schists in the Alps has been much changed by the orogenetic pro- 

 cess (dynamic metamorphism). Original material and material 

 mechanically produced at a later period, are often not to be separated 

 from one another. 



Besides these, the Alps present other difficulties that stand 

 in the way of the recognition of a stratigraphical grouping of the 

 crystalline schists. The field-relations are trequently so intricate, 

 that often it is very difficult to decide what originally lay under 

 and what above ; and whether the enormous thickness, for 

 instance, of many gneiss-complexes, is real, or merely produced 

 by repetitions of the folding, the folds being concealed by 

 cleavage. 



It follows that, if, on the basis of petrographical relations, a gene- 

 ral stratigraphy of the crystalline schists is to be attempted, this 

 must never take place as the result of observations made in plicated 

 regions of the earth' s crust ; districts must rather be chosen which 

 are not influenced by disturbances of the, Alpine character. In the 

 question of the stratigraphy of true crystalline schists, the Alpine 

 geologist is not in the position to furnish material of essential 

 value ; he must rather wait for the results of the workers in 

 other regions, in order to be able to apply them to his own 

 district. The dislocations of fractured regions have, in the main, 

 left unaltered the constitution of the tocks. There, then, the 

 crystalline schists can be studied in their unaltered condition. 

 There also they lie in flatter and more regular bedding ; and a 

 stratigraphical sequence is sooner to be found than in the Alps. 



ON THE ORIGIN OF THE PRIMITIVE 

 CRYSTALLINE ROCKS} 



T N this paper the author briefly summarizes the ideas prevailing 

 on the origin of the crystalline schists, and throws a doubt 

 on the current opinion that the primitive rocks have been formed 

 by the direct crystallization of their constituents. He divides 

 his treatise into two parts : (1) stratigraphical considerations ; 

 (2) the mode of association of the component minerals. 



(1) Stratigraphical Considerations.— The primitive crystalline 

 rocks form the fundamental floor upon which lie the earlier 

 detrital deposits, their schistosity being often parallel to the 

 stratification of the latter. 



Although composed mainly of acid gneisses, the primitive 

 rocks present countless variations in chemical and mineralogical 

 composition ; they include very basic representatives, such as 

 the amphibolites, pyroxenites, peridotites, cipolines, and dolo- 

 mites, <&c. These intercalations are always parallel to the 

 schistosity : they form elongated lenticular patches, of which 

 the greater axis is in the direction of the general banding. 



At the same time, their relative homogeneity in composition 

 is shown by comparison of sequences established, not only j n 

 Europe, but also in the United States and the rest of the world. 

 Acid gneisses predominate at the base ; then come frequent 

 intercalations of mica-schists and leptynites, with which are 



1 " Sur l'Origine des Terrains Cristallins Primitifs," by M. A. Michel- 

 LeVy, Bull. Soc. Geol. France. 3e serie, t. xvi. p. 102, 1888. Published by 

 the International Geological Congress in London, 1888. (Abstracted froTi 

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



