40 THOMAS STEERY HUNT ON A NATUEAL SYSTEM IN 



persilicate. With these sesquioxyd bases, which also iuclude chromic aud maugauio 

 oxyds, is ranged zircouia, since, notwithstaudiug the quadrivalence of zirconium, the rela- 

 tions of its oxyd in silicates are such as to place it at the side of alumina. Boric, titanic, 

 uiobic aud tantalic oxyds, all of which are found in silicates, are ranged, as already stated, 

 with silica, which they are regarded as replacing. 



§ 3t. Inasmuch as zircouia and chromic and manganic oxyds are but exceptionally 

 present in silicates, and fen'ic oxyd, though more commonly found than they, is much 

 less frequent therein than the alumina which it sometimes replaces, it maybe said that it 

 is essentially the relations of ahimina to the protoxyds and to silica which we are now 

 called to consider. Native silicates may be divided into those with aud those without 

 alumina, the latter division constituting the iirst suborder — Prulosilicate. Again, the 

 aluminiferous silicates either contain combined protoxyds, constituting the second sub- 

 order — Profopersilicate ; or are without protoxyds, making the third suborder — Persilicate. 

 The presence or absence of combined water, it being an element widely diflused in 

 nature, is of subordinate importance in the study of the silicates. Upon the general 

 distribution of silica and alumina in the crust of the earth, and the relations of these to 

 each other, to protoxyd bases, and to igneous and aqueous solvents, is based the whole 

 genetic history not only of the three suborders of silicates, but of quartz aud the non- 

 silicated oxyds. 



The affinities which determine the nearly contemporaneous formation of protosilicates 

 aud of protopersilicates are displayed in many different and unlike conditions which merit 

 especial consideration. This distinction is well seen in the basic crystalline rocks, wherein 

 pyroxene and chrysolite, often with magnetite, are found side by side with feldspars. 

 Whether this separation, which may be supposed to have taken place in a plutonic magma, 

 was effected with or without the intervention of water, is immaterial to our present 

 inquiry, since we know that the chemical aifinities involved lead to similar results alike 

 in the presence and the absence of water, and through a wide range of temperature. 

 Thus, a slow cooling from igneous fusion, as shown in the experiments of Fouqué and 

 Michel Levy, enables us to separate from the same basic magma successively chrysolite, 

 magnetite, pyroxene and feldspar. That similar affinities come into play in presence of 

 water at elevated temperatures is shown by the minerals in concretionary granitoid veins, 

 where wollastonite, amphibole and pyroxene are found crystallized with feldspars, 

 scapolite, micas, garnet and epidote, often with free quartz on the one hand and with mag- 

 netite, spinel and coriindum on the other. 



§ 38. A similar differentiation of protosilicates aud protopersilicates is presented in the 

 secretions from basic rocks. In the veins aud géodes found in such rocks are seen, side 

 by side, the protosilicates, pectolite, okenite, datolite and apophyllite, and the protoper- 

 silicates represented by pr(^hnite, epidote, and the various zeolites, and more rarely by 

 orthoclase and albite, by garnet aud tourmaline, — both quartz aud magnetite being also 

 present. The same distinction is observed in the products now forming in the channels 

 • of certain thermal springs, where pectolitic and zeolitic silicates are associated. Nor is the 

 differentiation less marked when, as in the experiments of Daubrée, water at a high tem- 

 perature acts upon glass. A protosilicate allied to okenite is then formed, together with 

 pyroxene and c|uartz, the alkaliferous solution retaining silica, with a portion of alumina. 

 From similar superheated solutions, holding other proportions of Ihcse same elements, 



