202 CHEMISTRY OF THE EARTH. 



silicate, iu which liuie and magnesia are wanting, andpotasli is the pre- 

 dominant alkali, (§ 28.) In such sediments as these just enumerated 

 we find the representatives of eruptive rocks like peridotite, phonolite, 

 leucitophyre, and similar rocks, which are so many exceptions in the 

 basic group of Bunsen. As, however, they are represented iu the sedi- 

 ments of the earth's crust, their appearance as exotic rocks, consequent 

 upon a softening and extravasation of the more easy liipiefiable strata 

 of deeply buried formations, is readily and simply explained. 



§ 44. In this connection a few words may be said about the popular 

 notion wliieh niakes granite the substratum of all stratified formations, 

 and even identities it with the supposed primitive crust of the globe. 

 That this crust is everywhere concealed beneath its own ruins, and, more- 

 over, that its composition must have been very different from granite, 

 we have endeavored to explain, (§ 10.) The Laurentian, the oldest known 

 system of rocks, includes iu its vast volume great interstratitied masses 

 of gneiss, often closely resembling granite, and it is extremelj^ probable 

 that these, softened and extravasated, may form the eruptive granites 

 which break through more recent systems of strata. These granitic 

 gneisses are, however, clearly stratified, and hold, moreover, intercalated 

 beds of quartzite and of limestone, often of great volume, and including- 

 the remains of an animal organism — the Eozoon Canadense. The pre- 

 dominance of feldspar, which gives the granitic character to the alumi- 

 nous rocks of early periods, has already been explained in § 29 as result- 

 ing from the great abundance of combined alkalies in these ancient 

 rocks. Tlie presence of quartz, an essential element alike in gneiss and 

 granite, would suffice to show that granite is iu all cases a secondary or 

 derived rock, formed under aqueous influences — even had Sorby not 

 shown that the minute crystal-cavities in the quartz of granitic rocks 

 contain liquid water which must have been introduced at the time ot 

 crystallization. Quartz has not only never been met with as a result ot 

 igneous fusion, but it is clearly shown by the exi^eriments of Rose that a 

 heat even much less than that required for the fusion of quartz destroys 

 it, changing it into a new substance, which differs both in chemical and 

 physical properties from quartz. AVe have pointed out in § 16 the chemi- 

 cal process by which it may be supposed that silica was set free from 

 the primitive silicated mass, under conditions which would permit its 

 conversion into quartz. 



§45. The rocks mentioned iu preceding sections are, as regards their 

 geognostical relations, divided into stratified or indigenous and erupted 

 or exotic rocks, the latter being- looked u[)on as the results of the soft- 

 ening and displacement of the former. Besides these, it is necessary to 

 distinguish a third kind of rock-masses, which, like the latter, occupy 

 fissures iu previously-formed rocks, but are unlike them iu origin, and 

 have been deposited from aqueous solutions. The most familiar form of 

 these is met with iu the vein-stones of quartz, calcite, barytine, and fluor, 

 which are often the gaugue of metallic ores. A careful study of the 

 various kinds of veins and their relations leads us, however, to admit 

 that almost all the mineral species which occur in the j)receding classes 

 of rocks may exist in vein-stones, which, from the mode of their produc- 

 tion, we have designated endogenous I'ocks. Calcareous veins in the 

 Laurentian rocks may contain all the mineral species of indigenous lime- 

 stones, and quartzo-feldspathic veins are made up of aggregates which 

 are familiarly designated as granites. To these, in fact, belong all those 

 so-called granitic veins which are marked by containing fine crystalli- 

 zations or rare mineral species. When, as is often the case, these marks 



