OEIGIN OF CRYSTALLINE ROCKS. 65 



groups must be added the non-alumiuous silicates, including hornblende, pyroxene, ensta- 

 tite and chrysolite, and the hydrous magnesiau species, serpentine and talc. Besides these 

 are free silica, generally as quartz, free oxyds, including the spinel and corundum groups, 

 which, together with the carbonates, make up the essential parts of the crystalline rocks. 



§ 129. Eock-masses, and the mineral species which compose them, present variations in 

 time, as we find in tracing the history of the great successive groups of crystalline strata ; 

 and they moreover show local changes, as seen in different parts of their distribution in the 

 same geological group. As regards the causes of these variations, very much remains to 

 be discovered by the patient collection and recording of facts concerning the associations 

 of mineral species, their artificial production, and their tr ansformations under the influences 

 of fire and water, and of solutions of potassic, sodic, calcareous and magnesian salts. The 

 instability of silicated compounds of igneous origin in the presence of water and watery 

 solutions, so widely diffused through nature, is the warrant for a general aqueous hypo- 

 thesis ; while, on the other hand, the derivation of stable mineral species, under such 

 influences, from matters of igneous origin justifies us in assuming for these species an 

 igneous starting point. 



Igneous fusion destroys the mineral species of the crystalline stratified rocks, and 

 brings them back as nearly as possible to the primary irudifferentiated lûaterial. Fire is 

 the great destroyer and disorganizer of mineral as well as of organic matter. Subterra- 

 nean heat in our time, acting upon buried aqueoixs sediments, destroys carbonates, sul- 

 phates, and chlorides, with the evolution of acid gases and the generation of basic 

 silicates, and thus repeats in miniature the conditions of the ante-ueptunian chaos, 

 with its surrounding acidic atmosphere. On the other hand, each mass of cooling igneous 

 rock in contact with water begins anew the formative process. The hydrated amorphous 

 product, palagonite, is, if we may be allowed the expression, a sort of silicated protoplasm, 

 which, in its differentiation, yields to the solvent action of water the crystalline silicates 

 which are the constituent elements of the crenitic rocks, leaving, at the same time, a more 

 basic residuum abounding in magnesia and iron-oxyd, and soluble not by crenitic but by 

 sub-aerial action. Palagonite, or some amorphous matter resembling it, probably marks 

 a stage in the sub-aqueous transformation of all igneous rocks, though only under special 

 conditions does this unstable, hydrous substance form appreciable masses. lu all cases, 

 igneous matter, of primary or of secondary origin, serves as the point of departure. 



According to the proposed hypothesis, which derives rocks of the granitic type, com- 

 posed essentially of quartz and feldspars, by aqueous secretion from a primary igneous 

 and quartzless mass, it would follow that the highly basic compoiind, assumed by Bunsen 

 to represent the typical pyroxenic or basaltic rock (§ 24), would be the above mentioned 

 insoluble residuum ; and that less basic A^arieties of similar rocks would correspond to 

 portions of the same primary mass, less completely exhausted by lixiviation, and conse- 

 quently approaching in composition to admixtures of the basaltic and granitic types, as 

 maintained on other grounds by Bunsen himself 



§ 130. The principles which have been enumerated in the preceding pages will, it is 

 believed, lead the way, not only to a natural system of mineralogy, but to a natural system 

 of classification of crystalline rocks, considered with regard alike to their chemical compo- 

 sition, their genesis, and their geological succession. A valid hypothesis for the crystalline 



Sec. UL, 1884. 9. 



