Nov. 9, 1 871 



NA TURE 



roxenes, presupposes the existence of crystalline rocks, whose 

 generation this neptunist never attempts to explain, but takes his 

 starting-point from a plutonic basis." 



I then asserted that the problem to be solved in regional meta- 

 morphism is the conversion of sedimentary strata, "derived by 

 chemical and mechanical agencies from the ocean waters and 

 pre-existing crystalline rocks into aggregitions of ciystalline sili- 

 cates. These metamorphic rocks, once formed, are liable to 

 alteration only by local and superficial agencie.s, and are not, like 

 the tissues of a living organism, subject to incessant transfor- 

 in^tinnc. the pseudomor[)hisin of Bischof "* 



1 lijil not, at that time, seen the essay by Delesse on pseudo- 

 morphs already referred to, published in 1859, in which he main- 

 tained views similar to those set forth by me in 1853 and 1S60, 

 dec'aring that much of what had been regarded as pseudomor- 

 phism had no other ba^is than the observed associations of 

 minerals, and that often "the so-called metamorphism tinds its 

 natural explanation in envelopment." These views he ably and 

 ingeniously defended by a careful discussion of the whole range 

 of facts belonging to the history of the subject. 



My own expression of opinion on this question, in 1853, had 

 been privately criticised, and I had been charged with a want of 

 comprehension of the question. It was, therefore, with no small 

 pleasure, that I not only saw my views so ably supported by 

 Delesse, but read the language of Carl Friedrich Naumann, who 

 in 1S61 wrote to Delesse as follows, referring to his essay just 

 noticed : — 



" You have rendered a veritable service to science in restricting 

 pseudomorphs to their true limits, and separating what had been 

 erroneously united to them. As you have remarked, envelop- 

 ments have, for the most part, nothing in common with pseudo- 

 morphs, and it is inconceivable that they have been united by so 

 many mineralogists and geologists. It appears to me, moreover, 

 that they commit an analogous error when they regard gneisses, 

 amphibolites, &c., as being, all of them, the results of metamor- 

 phic epigenesis, and not original rocks. It is precisely because 

 pseudomorphism has been so often confounded with metamor- 

 phism that this error has found acceptance. I only admit a 

 pseudomorph where there is some crystal the form of which has 

 been preserved. There are very many metamorphic substances 

 which are, in no sense of the word, pseudomorphs. Had the 

 name of crystalloid been chosen instead of pseudomorph, this 

 confusion would certainly have never found its way into the 

 science. I think, with you, that the envelopment of two mine- 

 rals is most generally explained by a contemporaneous and original 

 crystallisrition. Secondary envelopments, however, exist, and 

 such may be ca'Ied pseudomorphs or crystalloids, if they repro- 

 duce exactly the form of the crysal enveloped, whether this last 

 still remains, or has entirely disappeared. "+ 



It is unnecessary to remark that the view of Delesse and 

 Naumann — viz. : that the so-called cases of pseudomorphism, on 

 which the theory of metamorphism by alteration has been built, 

 are, for the most part, examples of association and envelopment, 

 and the result of a contemporaneous and original crystallisation — 

 is identical with the view suggested by Sche^rer, a:id generalised 

 by myself long before, when, in 1S53, I .'■ought to explain the 

 phenomena in question by **the association and crystallising 

 together of homologous and isomorphous species." 



Later in 1862, I wrote as follows : — 



" Pseudomorphism, which is the change of one mineral species 

 into another, by the introduction or the elimination of some 

 element or elements, presupposes metamorphism [i.e., meta- 

 morphic or crystalline rocks), since only definite mineral species 

 can be the subjects of this process. To confound metamorphism 

 with pseudomorphism, as Bischof and others after him have 

 done, is therefore an error. It may be further remarked, that, 

 although ceitain pseudomorphic changes may take place in some 

 mintral species, in veins and near the surface, the alteration of 

 great masses of silicated rocks by such a process is as yet an 

 unproved hypothes's. "+ 



Thus this unproved theory of pseudomorphism, as taught by 

 Bischof, does not, even if admitted to its (uUest extent, advance 

 us a single step toward a solution of the problem of the origin of 

 the various silicates, which, singly or intermingled, make up 

 beds in the crystalline schists. Granting, for the sake of argu- 

 ment, that serpentine results from the alteratioa of olivine or 



• Amer. Jour Sci.. 11. xxx- ,35. 

 t Bull. Soc. Geol. de France, II. xv.ii. 678. 



t Descriptive Catalogue, Crystalline Rocks of Canada, p. So, London 

 Exhibition, 1862 : also Dublin Quar. Jour., July 1863, and Amer. Jour. 



labradorite, and steatite or chlorite from hornblende, the origin 

 of these anhydrous silicates, which are the subjects of the sup- 

 p 'sed change, is still unaccounted for. The explanation of this 

 shortsightedness is not far to seek ; asalrea'^y remarked, Bischof, 

 although a professed neptunist, starts Irom a plutonic bas's. 

 When the epigentc origin of serpentine and its related rocks 

 was first taught, these were regarded as eruptive and unstratified, 

 and it was easy to imagine intruded masses of dioriticand feld- 

 spathic rocks, which had become the subjects of alteration. As, 

 however, the progress of careful investigation in the field has 

 shown the stratified character of these serpentines, diallag:-rocks, 

 steatites. Src, and th^ir inVrcihtion among limestones, argilhtes, 

 quartzitts, gneisses, and mica-schists, and even among feldspathic 

 and homblendic strata, we are forced to reject, with Naumann, 

 the notion of their epigenic derivation, and to regard them as 

 original rocks. 



This view brings us face to face with the problem of metamor- 

 phism as defined by me in 1S60* (see ante). We must either 

 admit that these crystalline schists were created as we find them, 

 or suppose that they were once sands, clays, marls, &c. ; in a 

 word, sediments of chemical and mechanical origin, which by a 

 subsequent process have been consolidated and crystallised. 

 Whence, then, come these silicates of magnesia, lime, and iron, 

 which are the sources of serpentine, hornblende, steatite, chlorite, 

 &c.? This is the question which I proposed in that same year, 

 when, after discussing the results of ray examinations of the 

 tertiary rocks near Paris containing layers of a hydrous silicate 

 of magnesia related to talc in composition, among unaltered 

 limestones and clays, I remarked that it is evident " .such silicates 

 may be formed in basins at the eirth's surface, by reactions 

 between magnesian solutions and dissolved silica ; " and, after 

 some further discussion, said, " further inquiries in this direction 

 may show to what extent certain rocks composed of calcareous 

 and magnesian silicates may be directly formed in the moist wdy."*h 

 Subsequently, in a paper on " The Origin of some Magnesian 

 and Aluminous Rocks," printed in the '' Canadian Naturalist " 

 for June iS6o,J I repealed these considerations, referring 

 to the well-known fact that silicates of lime, magnesia, and 

 iron-o.fyd are deposited during the evaporation of natural 

 water.s, including those of alkaline springs and of the Ottawa 

 River. Having described the mode of occurrence of the mag- 

 nesian silicate sepiolite, in the Paris basin, and the related 

 quincife, containing some iron-oxyd and disseminated in lime- 

 stone, I suggested that while steatite has been derived from a 

 compound like sepiolite, the source of serpentine was to be 

 sought in another silicate richer in magnesia ; and, moreover, that 

 chlorite, unless the result of a subsequent reaction between clay 

 and carbonate of magnesia, was directly formed by a process 

 analogous to that which (according to Scheerer) has, in recent 

 times, caused the deposition from w-aters of neolite, a hydrous 

 alumino-magnesian silicate, approaching to chlorite in composi- 

 lion,§ "the type of a reaction which formerly generated beds of 

 chlorite in the same way as those of sepiolite or talc. ' Delesse, 

 subsequently, in i86i, in his essay on Rock-Metamorphism, in- 

 sisted upon the sepiolttes or so called magnesian marls, as pro- 

 bably the source of steatite, and suggested the derivation of ser- 

 pentine, chlorite, and other related minerals of the crystalline 

 schists, from deposits approaching these marls in composition.il 

 He recalled, also, the occurrence of chromic oxyd, a frequent 

 accompaniment of these magnesian minerals, in the hydrated 

 iron ores of the same geological horizon with the magnesian 

 marls in France. Delesse did not, however, attempt to account 

 for the origin of these deposits of magnesian marls, in explana- 

 tion of which I afterwards verified Bischof 's observations on the 

 sparing solubility of silicate of magnesia, and showed that silicate 

 of soda, or even artificial hydrated silicate of lime, v.'hen added 

 to waters containing magnesian chlorid or sulphate, gives rise, 

 by double decomposition, to a very insoluble magnesian silicate. ir 



To explain the generation of silicates like labradorite, scapo- 

 lite, gamite, and saussurite, I suggested that double aluminous 

 silicates allied to the zeolites might have been formed, and sub- 

 sequently rendered anhydrous. The production of zeolitic 

 minerals observed by Daubree at Plombieres and Luxeuil by the 

 action of a silicated alkaline water on the masonry of ancient 

 Roman baths, was appealed to by way of illustration. It h.ad 



* Amer. Jour. Sci., II. xitx. 135. 



t Ibid.,II. xxix. 284: also II. xl. 49. 



t Ibid., II. xxxii. j86. 



§ Pog. Annal., Ixxi. 288. 



tl Etudes surle Weumorphisme, 4to. pp. 91. Paris, 1861. 



1 Amer. Jo 



.Sci., n 



