224 METAMORPHIC ROCKS. 



gneiss ; complete pastiness, completely destroying stratification, makes 

 granite. The principal arguments for this view may be briefly stated 

 as follows : * 



1. In many localities in mountain-regions, and nowhere better than 

 in the Sierra of California, every stage of gradation may be observed 

 between clayey sandstones and gneiss, and between -gneiss and granite. 

 So perfect is this gradation, that it is impossible to draw sharply the 

 distinction. Even geologists who believe that granite is the primitive 

 rock have been compelled to admit that there is also a metamorphic 

 granite, scarcely distinguishable from primitive granite. 



2. Not only gneiss, but even granite, is sometimes interstratified 

 with undoubted sedimentary rocks, f 



3. Chemists recognize two kinds of silica, viz., an amorphous va- 

 riety of specific gravity 2*2, and a crystallized variety, specific gravity 

 2 -6. These two varieties differ from each other not only in density, 

 but also in chemical properties, the former being much more easily 

 attacked by alkalies than the latter. By solidification from fusion (dry 

 way) only the variety of specific gravity 2*2 can be formed, while the 

 variety 2'6 is formed only by slow deposit from solution (humid way). J 

 Now, the quartz of granite is always of the variety 2*6, and therefore 

 must have been formed in presence of water. 



4. Crystals of quartz, hornblende, and mica, are frequently formed 

 in Nature by the humid process, as, for example, in metamorphic rocks ; 

 and have also been artificially formed by the same process by Daubree, 

 Senarmont, and others, as already stated (p. 221) ; but they have never 

 been formed artificially by the dry way. 



5. In nearly all rocks and minerals microscopic cavities are found 

 indicating the conditions under which crystallization or solidification 

 took place. If crystals are formed by sublimation, they contain vacu- 

 ous cavities. If they are formed by solidification from fusion (dry way), 

 and if gases or vapors are present, they may contain vapor-blebs ; but, 

 if they crystallize slowly from a glassy magma, they contain spots of 

 glassy matter, or glass cavities or inclusions, as in slags and lavas. If 

 they are formed by crystallization from solution, then they h&ve fluid 

 cavities, or liquid inclusions, as they are now usually called. Now, not 

 only are these fluid cavities found in metamorphic rocks, but also in 

 the quartz and feldspar of granite. " A thousand millions of these 

 microscopic cavities in a cubic inch is not at all unusual ; and the in- 

 closed water often constitutes one to two per cent of the volume of the 



* Rose, Philosophical Magazine, xix, p. 32 ; Delesse, Archives des Sciences, vol. vii, p. 

 190; Hunt, American Journal of Science and Arts, new series, vol. i, pp. 82, 182. 



f Dana, American Journal of Science, vol. xx, p. 194, 1880. 



% Recently quartz, specific gravity 2*6, has been formed under peculiar conditions by 

 dry fusion, American Journal of Science, vol. xvi, p. 155, 1878. 



