Sept. 27, 1888] 



NATURE 



5'9 



ON CRYSTAL! INE SCHISTS} 



I. 



§ I. ASa preliminary to the study of the schistose or stratiform 

 f^ crystalline rocks, it is desirable to consider the wider 

 question of the origin of crystalline rocks in general, which are 

 •often named Primary or Primitive Rocks to distinguish them 

 from those derived therefrom by mechanical or chemical dis- 

 integration. The designation of "crystalline rocks " is defective, 

 inasmuch as we find, associated with masses having a right to 

 this title, and geologically confounded with them, other rocks, 

 such as serpentine, obsidian, perlite, and others, which are not 

 crystalline in character, but colloidal, or, to use the designation 

 of Breithaupt, porodic. The primary rocks, then, including 

 both crystalline and porodic masses, may be divided geologically 

 into three categories : — 



(1) Masses rmre or less distinctly stratiform, including the 

 fundamental granite, gneisses, micaceous and hornblendic 

 schists, and all others formed, according to the views of the 

 Wernerian school, by slow deposition in an aqueous liquid 

 at the earth's surface. These we call Indigenous Rocks. 

 (2) Masses which have strong mineralogic resemblances with 

 the last, but appear to have been formed by slow deposition 

 among pre-existing rocks, in which they occur as veins or 

 ■secondary masses, and which we have consequently designated 

 Endogenous Rocks. (3) Masses which have resemblances, both 

 mineralogic and geognostic, with the endogenous rocks, but are 

 distinguished therefrom by the fact that they appear to have 

 attained their present position not by slow deposition, but as the 

 result of displacements which took place while they were in a 

 more or less liquid or plastic state. These masses, which we 

 designate Exotic Rocks, are, as we shall endeavour to show, 

 to be regarded (whatever their geological age) either a~ more or 

 Jess modified portions of the original plutonic material of the 

 globe, or as displaced portions of indigenous or endogenous 

 rocks, and thus in either case belong to the primary class. 



§ 2. Without taking into account those who, like Eehmann 

 in the last century, maintained that the indigenous crystalline 

 masses, which he called primitive rocks, were created as we 

 now see them, we may say that the geologists of our own time 

 are divided into two classes : those who admit for the indigenous 

 rocks (1) an igneous or plutonic origin, (2) an aqueous or neptu- 

 nian origin. Among the plutonists properly so called there are, 

 moreover, two schools, one of which regards the foliated struc- 

 ture which characterizes the crystalline schists as due to the 

 lamination of an igneous mass exposed to strong pressure during 

 its extrusion through the already solidified terrestrial crust. For 

 this school, in fact, the crystalline schists, not less than the 

 granites, the trachytes, and the basalts, are eruptive rocks. This 

 manner of explaining the oiigin of the crystalline schists, ad- 

 vanced by Poulett Scrope in 1825, and since frequently resusci- 

 tated, we have named the exoplutonic or volcanic hypothesis. 

 For the other plutonist school, these same crystalline schists are 

 the products of the consolidation, beneath a crust already formed 

 by superficial cooling, of the igneous matter of the globe ; the 

 schistose structure being the result either of currents established 

 in the still liquid and heterogenous material, or of a segregation 

 therein during cry tallization. To the views of this second 

 plutonist school we have given the name of the cinioplutoiiic 

 hypothesis. 



§ 3. The neptuni>ts are also divided into several schools. 

 Werner and his disc : ples believed that the crystalline rocks, both 

 granitic and schistose, had been successively deposited from a 

 universal ocean, which they imagined to have been a chaotic 

 liquid holding in solution the elements of all the primitive rocks. 

 We have called this derivation by slow crystallization from a 

 primordial liquid chaos, the chaotic hypothesis. In this purely 

 neptunian hyp ^thesis, the action of a heated interior of the earth 

 did not enter, but certain plutonists, admitting this notion, have 

 imagined a thermochaotic hypothesis. This was advanced by 

 l'oulett Scrope, in 1825, as the complement of his exoplutonic 

 hypothesis, and subsequently sustained by De la Beche and 

 Daubree. 



Another neptunist school, which also held plutonic views, 

 was that of Hutton, who supposed that the crystalline rocks now 

 known to us have been formed by the consolidation and crystal- 

 lization, through the agency of internal heat, of sediments 



1 Translated by the author from his essay on " Les Schistes Cristallins." 

 presented to, and published in French by, the International Gaol >gical 

 congress in London, 1888. The footnote to § 5 has bsen ad Jed in translating 



arranged by water at the bottom of the seas, these sediments 

 being the detritus either of endoplutonic or of exoplutonic rocks. 

 The defect of this explanation, which we call the met amorphic 

 hypothesis, is that it does not take into account the chemical 

 changes suffered by most silicated mineral species during the 

 process of disintegration of the crystalline rocks and their con- 

 version into sands and clays. The production of species such as 

 the feldspars, the micas, hornblende, &c, as the result of a re- 

 crystallization of sediments which do not contain the elements of 

 these minerals, demands the additional supposition of chemical 

 changes brought about either by substitution or by simple addi- 

 tion. In this manner, attempts have been made to explain sup- 

 posed transformations, often very surprising, among which may 

 be noted, not only the conversion of siliceous and argillaceous 

 sediments into feldspathic and hornblendic rocks, but that of 

 limestones into gneiss and other feldspathic and siliceous rocks 

 and also the conversion of these, as well as of diabases and 

 diorites, into serpentine, or into crystalline limestone. This view, 

 which we have called the metasomatic hypothesis, is, in the 

 minds of many geologists, confounded with the metamorphic 

 hypothesis of Hutton, of which it is, to a certain extent, the 

 indispensable complement. 



§ 4. Of all these hypotheses, that of Werner, which considered" 

 the primaeval chaos as a watery liquid holding in solution the 

 materials necessary for the formation of all the crystalline rocks, 

 appears to us the one nearest the truth. It is certain, however, 

 that in the present state of our chemical knowledge we cannot 

 admit the simultaneous existence of all these materials in solu- 

 tion, even at the elevated temperature supposed by the ther- 

 mochaotic hypothesis. We have, however, endeavoured to 

 reconcile with known facts the view that a great part of all 

 the primary rocks, including both the granites and the crys- 

 talline schists, have at one time been in the state of aqueous 

 solution, through the action of processes which have operated 

 without cessation from the Primary period. This explanation, 

 which we have elsewhere set forth in detail, after a critical 

 examination of the other hypotheses already mentioned, we have 

 named the crenitic hypothe.-is, from the Greek Kpfyt], fountain or 

 spring. 



Starting from the conception of a liquid globe of igneous 

 origin, the solidification of which commenced at the centre, we 

 find in its exterior portion — the last to solidify — the source of all 

 the known terrestrial rocks ; in other words, the veritable 

 mineral protoplasm. This material we suppose to have been, 

 from the time of its superficial cooling, exposed to the action of 

 water and the atmospheric gases, while it was at the same time 

 heated from below by the internal warmth, and penetrated to a 

 greater or less depth by watery solutions. These, under the 

 influence of the existing thermal differences, must have estab- 

 lished a circulation between the surface and the deeper portions 

 of the protoplasmic mass, which, as the result of crystallization 

 and cooling, had already become porous. From the abundant 

 outflow of thermal waters thus produced is derived the name 

 "crenitic," given alike to the mineral deposits formed by them 

 and to the present hypothesis. The action of these waters, 

 removing from the protoplasmic material silica, alumina, and 

 potash, and bringing to it at the same time lime, magnesia, and 

 soda, must have necessarily altered by degrees the composition of 

 this porous mass, heated from below, penetrated by aqueous 

 solutions, and rendered more or less plastic in parts. In the 

 changing mass, moreover, took place processes of crystallization, 

 followed by partial separations determined by differences in 

 specific gravity between the species thus formed. In this way 

 were produced various types of plutonic rocks, which may justly 

 be called Primary, since they are more or less modified portions 

 of the original protoplasmic material. 



§ 5. The dissolving action of the circulating waters continued 

 without interruption from a very remote period in the history of 

 the globe, and, extending eventually to depths equal to very 

 many kilometres, while giving rise to the immense thickness of 

 crenitic rocks which cover the surface of the protoplasmic mass, 

 must necessarily have effected a great diminution therein. This 

 decrease of volume beneath the crenitic covering must have 

 resulted in movements giving rise to the more or less marked 

 corrugations everywhere met with in the earlier layers of the 

 crenitic envelope — movements which have continued, though 

 with decreasing force, through all geological periods. Moreover, 

 the accumulated weight, alike of crenitic deposits and of 

 mechanical sediments, would bring about at length the displace- 

 ment, in a plastic state, of poriions of the primitive mass, as well 



