MINERALOGY ; WITH A CLASSIFICATION OF SILICATES. 41 



crystals of orthoclase, of albite, of analcite and other zeolites, with qiaartz and tridymite, 

 have been obtained. 



§ 39. To these considerations on the genesis of the two suborders of protosilicates and 

 protopersilicates must be added the history of their decay and transformations. It is 

 known that the subaerial decay of protosilicates through the action of atmospheric waters 

 effects their complete decomposition. The lime and magnesia of amphibole, pyroxene, and 

 chrysolite, are thereby dissolved, together with a large proportion of the silica itself; a part 

 of this, howeA-er, according to Ebelmen, remains behind, together with the iron changed 

 from a ferrons condition to that of ferric hydrate. In the subaerial decay of such proto- 

 persilicates as the feldspars and closely related species, the protoxyd bases, chiefly alkalies 

 and lime, pass into solution, together with a large part of the silica ; and the alumina 

 united with the remainder of the silica, and with a portion of water, remains as an insol- 

 uble compound, which in many cases has the composition of kaolin. This decay of the 

 feldspars plays an important part in terrestrial chemistry. The process is slow and gra- 

 dual, and the feldspar softens and becomes disintegrated before the loss of protoxyds is 

 complete, so that the clays thus formed still retain, in many cases, a portion of alkali 

 which may amount to two or three hundredths. The decomposition of the more basic 

 feldspars and feldspathic minerals will be considered farther on, as also the genesis of 

 various micaceous and colloid or clay-like persilicates. 



§ 40. From the subseqirent transformation of clays more or less completely deprived 

 of alkalies, are apparently derived, in many cases at least, muscovitic micas and toiirma- 

 line, together with the crystalline persilicates, kaolinite, pyrophyllite, andahisite, cyanite, 

 fibrolite and related species. The micas just mentioned are more stable under atmospheric 

 influences than the feldspars, while those which like phlogopite and biotite abound in 

 protoxyds, yield readily to decay. The harder and gem-like protopersilicates resist to a 

 greater extent this process, and the more common species of these — garnet, epidote and 

 tourmaline — are found unchanged in sands with persilicates such as andalusite, topaz and 

 zircon, and with cjuartz, corundum, spinel and menaccanite. 



Thus the natural processes of subaerial decay destroy the protosilicates, and transform 

 the predominant types of protopersilicates, either into more highly aluminous and more 

 stable types of the same suborder, or into persilicates ; in all cases with the separation of 

 the elements of protosilicates, that is to say, of silica and protoxyd bases. Of these, while 

 iron is separated in an insoluble ferric state, the alkalies and lime, as well as the magnesia 

 of the protosilicates, pass into the condition of carbonates, the silica being liberated in a 

 soluble form. 



§ 41. The minerals of granitoid A'eins, and those secreted in basic rocks, show that a 

 distinct subterranean process of solution of silicated compounds goes on under conditions as 

 yet imperfectly understood, and probably under the influence of thermal alkaline waters, 

 From the matters thus dissolved come, not only the protopersilicates deposited in the forms 

 of zeolites, and of feldspars, micas, garnets and tourmalines ; but also the calcic and alkaline 

 protosilicates which take the forms of pectolite, xonaltite, okenite, apophyllite and wol- 

 lastonite. Such protosilicate solutions, coming in contact with atmospheric carbonic 

 dioxyd, would be decomposed with separation of carbonates, but with dissolved magnesian 

 salts would yield by double exchange silicates such as sepiolite, talc, serpentine, enstatite, 

 chrysolite and their associated amphiboles and pyroxenes. It will be remembered that 



Sec. III., 1885. U. 



