Nov. 2, 1 871 J 



NATURE 



17 



seilimentary rocks, is not the only one, and is inailequate to ex- 

 plain the production of many of the varieties of stratified silicated 

 rocks, sucli are serpentine, steatite, liornblende, diall.ige, chlo- 

 rite, piiiiie, nnd labradorite, all of which mineral species form 

 rock masses by themselves, frequently almost without admixture. 

 No geological student will now question that all of these rocks 

 occur as membsrs of stratified formations. Moreover, the 

 manne-^ in which serpentines are found interstratilietl with steatite, 

 chlorite, argiilile, diorile, hornblende, anel feldspir rocks, and 

 these, in their turn, with quartzites and orthoclaiie rocks, is such 

 as to forbid the notion that these various materials have been 

 deposited, with their present composition, as mechanical sedi- 

 ments from the ruins of pre-existing rocks ; a hypothesis as un- 

 tenable as that ancient one which supposed them to be the direct 

 results of plutonic action. 



There are, however, two other hypotheses which have been 

 proposed to explain the origin of these various silicated rocks, 

 and especially of the less abundant, and, as it were, exceptional 

 species just mentioned. Tlie first of these supposes that the 

 minerals of which they are composed have resulted from an 

 alteration of previously existing minerals, often very unlike in 

 composit on to the present, by the taking away of certain elements 

 and the addition of certain others. This is the theory of meta- 

 morphism by pseudomorphic changes, as they are called, and is 

 the one taught by the now reigning school of chemical geologists, 

 of which the learned and laborious Bischof, whose recent death 

 science deplores, may be regarded as the great exponent. The 

 second hypothesis supposes that the elements of these various 

 rocks were originally depo5ited as, for the most part, chemically 

 formed sediments, or precipitates ; and that the subsequent 

 changes have been simply molecular, or, at most, confined in 

 certain cases to reactions between the mingled elements of the 

 sediments, with the elimination of water and carbonic acid. It 

 is proposed to consider briefly these two opposite theories, which 

 seek to explain the origin of the rocks in question respectively 

 by pseudomorphic changes in pre-existing crystalline rocks, and 

 by the crystallisation of aqueous sediments, for the most part 

 chemically-formed precipitates. 



Mineral pseudomorphism, that is to say, the assumption by one 

 mineral substance of the crystalline form of another, may arise 

 in several ways. First of these is the filling up of a mould left 

 by the solution or decomposition of an imbedded crystal, a pro- 

 cess which sometimes takes place in mineral veins, where the 

 processes of solution and decomposition can be freely carried on. 

 Allied to this, is the mineralisation of organic remains, where 

 carbonate of lime or silica, for example, fills the pores of wood. 

 When subsequent decay removes the woody tissue, the vacant 

 spaces may, in their turn, be filled by the same or another 

 species.'' Lathe second place, we may consider pseudomorphs 

 from alteration, which are the result of a gradual change in the 

 composition of a mineral species. This process is exemplified 

 in the conversion of feldspar into kaolin by the loss of its alkali 

 and a portion of silica, and the fixation of water, or in the change 

 of chalybite into llmonite by the loss of carbonic acid and the 

 absorption of water and oxygen. 



The doctrine of pseudomorphism by alterati.jn as taught by 

 Gustav Rose, Haidinger, Blum, Volger, Rammelsberg. Dana, 

 Bischof, and mmy others, leads them, however, to admit still 

 greater and more remarkable changes than these, and to maintain 

 the possibility of converting almost any silicate into any other. 

 Thus, by refcriing to the pages of Bischof 's Lchi bitch dcr Geo- 

 pwsie, it will be found that serpentine is said to exist as a pseudo- 

 morph after augite, hornblende, olivine, chondodrite, garnet, 

 mica, and probably also after labradorite, and even orthoclase. 

 Serpentine rock or ophiolite is supposed to have resulted, in 

 different cases, from the alteration of hornblende-rock, diorite, 

 granulite, and even granite. Not only silicates of protoxyds and 

 aluminous silicates are conceived to be capable of this transfor- 

 mation, but probably also quartz itself; at least, Blum asserts 

 that meerschaum, a closely related silicate of magnesia, which 

 sometimes accompanies serpentine, results from the alteration of 

 flint, while, according to Rose, serpentine may even be produced 

 from dolomite, which we are told is itself produced by the alteration 

 of limestone. But this is not all — feldspar may replace carbonate 

 of lime, and carbonate of lime feldspar, so that, according to 

 Volger, some gneissoid limestones are probably formed from 

 gneiss by the substitution of calcite for orthoclase. In this way 

 we are led from gneiss or granite to limestone, from limestone to 

 dolomite, and from dolomite to serpentine, or more directly from 

 granite, granulite, or diorite to serpentine at once, without pass- 

 *Hunt on the Silification of Fossils, Canadian Naturalist, N, S., I. 46. 



ing through the intermediate stages of limestone and dolomite, 

 till we are ready to exclaim in the words of Goethe : — 

 •' Mich .'ing5tiBt d.is Verfdngliche 

 Im widrigen Geschwatz, 

 Wo Nichts verharrct. Alles flielit. 

 Wo scho-.i verschwundcn was man si -l.l," 

 which we may ihus trin-Ute : — "I am vexed with the sophistry 

 in their cont ary jargon, where nothing endures, but all is fugi- 

 tive, and where what we see has already passeil a-A'ay. " 



By far the greater numl)er of cases on which this general 

 theory of p-^eudomorphism by a slow process of alteration iu 

 minerals, has been based, are, as I shall endj-avour to show, 

 examples of the phenomenon of mineral envtlopment, .'o well 

 studied by Deles>e in his essay on pseulomjrphs, * and may be 

 considered under two heids : — first, thit of symmetrical envelop- 

 ment, in which one miner d species is so enclosed within the 

 other that the two appear to form a sing'e crystalline individual. 

 Examples of this are seen wdien prisms of cyaniteare surrounded 

 by staurolite, or staurolite crystals completely enveloped in those 

 of cyanite, the vertical axes of the two prisms corresponding. 

 Similar cases are seen in the enclosure of a prism of red in an 

 envelope of green tourmaline, of allanite in epidote, and ol 

 various minerals of the pyroxene group in one another. The 

 occurrence of rauscovite in lepidolite, anl of margarodite in 

 lepidomalene, or the inverse, are well known examples, and, 

 according to Scheerer, the crystallisation of serpentine around a 

 nucleus of olivine is a simiUr case. This phenomenon of sym- 

 metrical envelopment, as reimrked by Delesse, shows it.self with 

 species which are generally isomorphous or homoeomorphous, and 

 of related chemical composition. Allied to this is the repeated 

 alternation of crystilline lamince of related spec'es, as in per- 

 thiie, the crystalline cleavable masses of which consist of thin 

 alternating layers of orthoclase and albi'e. 



Very unlike to the ab )ve are those cases of envelopment in 

 which no relations of crjstalline symmetry nor of similar chemi- 

 cal constitution can be traced. Examples of this kind are seen 

 in garnet crystals, the walls of which are shells, sometimes no 

 thicker than paper, enclosing in different cases crystalline car- 

 bonate of lime, epidote, chlorite, or quartz. In like manner, 

 crystalline shells of leucie enclose feldspar, hollow prisms of 

 tourmaline are filled with crystals of mica or w,th hydrous 

 peroxyd of iron, and crystals of beryl with a granular mixture of 

 orthoclase and quartz, holding small crystals of garnet and tour- 

 maline, a composition identical wi'h the enclosing granitic vein- 

 stone.t Similar shells of galen;te and of zircon, having the 

 external forms of these species, are also founJ fi led wi h calcite. 

 In many of these cases the p ocess seems to have been first the 

 formation of a hollow mould or skeleton crystal (a phenomenon 

 sometimes observed in salts crystallising from solutions), the 

 cavity being sometimes filled with other matters. Such a pro- 

 cess is conceivable in free crystals found in veins, as for example, 

 galenite, zircon, tourmaline, beryl, and some examples of garnet, 

 but is not so intelligible in the case of those garnets imbedded in 

 mica-schist, studied by Delesse, which enclosed within their 

 crystalline shells irregular masses of white quartz, with some 

 little admixture of garnet. Delesse conceives these and similar 

 cases to be produced by a process analogous to that seen in the 

 crystallisation of calcite in the Fontainebleau sandstone ; where 

 the quartz grains, mechanically enclosed in well-defined rhombo- 

 hedral crystals, equal, according to him, sixty-five per cent, of 

 the mass. Very similar to these are the crystalloids with the 

 form of orthoclase, which sometimes consist in large part of a 

 granular mixture of quartz, mica, and orthoclase, with a little 

 cassilerite, and in other cases, contain two thirds their weight of 

 the latter mineral, with an admixture of orthoclase and quartz. 

 Crystals with the form of scipolite, but made up, in a great 

 part, of mica, seem to be like cases of envelopment, in which a 

 small proportion of one substance in the act of crystallisation, 

 compels into its own crystalline form a large portion of some 

 foreign material, which may even so mask the crystallising 

 element that this becomes overlooked, as of secondary import- 

 ance. The substance which, under the name of houghite, has 

 been de'cribed as an altered spinel, is found by analysis to be 

 the mixture of vollknerite with a variable proportion of spinei, 

 which in some specimens, does not exceed eight per cent. , out 

 to w Inch, nevertheless, these crystalloids appear to owe their more 

 or less complete octohedral form. J 



( To be continued. ) 



* Anna'es des Mines, V. xvi. ji 7-392. 



t Report Geol. Survey of Canada, 1866, p. i8g. 



J Rpt. Geol. Sur. of Onada, 1666, pp. 189, 213. Am;r Jour. Sci ,111. 1. iS3. 



