METAMORPHISM. 709 



in which state limestone might have been pressed into fissures in 

 adjoining rocks so as to make a species of injected vein. The fis- 

 sures and openings in rocks formed while the metamorphism was 

 in progress, and the distinctness in most cases of the original planes 

 of lamination, are evidence that this plastic or semi-fused state was 

 not common in metamorphic operations. It may have been one of 

 the conditions requisite for the formation of granite, — a non-schist- 

 ose rock ; and the transitions from gneiss to granite, which are by 

 imperceptible gradations, may indicate different degrees of this state. 

 There may seem to be some difficulty in accounting for metamor- 

 phic results on the ground of the diversity of mineral species that 

 are produced. But, in the first place, the elements constituting these 

 species are few in number, — silica, alumina, potash, soda, lime, mag- 

 nesia and the oxyds of iron being all that are necessary for the 

 great majority of them ; in the second place, the material of sedi- 

 mentary strata is, to a large extent, nothing but pulverized metamor- 

 phic rocks, so that the metamorphism is often only a new crystal- 

 lization of the minerals already present ; and, in the third, the 

 organic remains, out of which many rocks have been largely made, 

 even the arenaceous and argillaceous, have contributed a variety of 

 ingredients, besides carbonate of lime, — the most important of 

 which are phosphoric acid and fluorine (p. 66). 



Some argillaceous beds consist largely of true clays, resulting from the decom- 

 position of feldspar or other aluminous minerals. But generally they are made 

 mainly of pulverized feldspar with quartz, as is proved by the presence of alka- 

 lies found by chemical analysis. When the alkalies are absent, as Hunt has 

 stated, metamorphism cannot produce feldspar, but may fill the slate with anda- 

 lusite, kyanite, or other non-alkaline species. 



The following table presents a general view of the composition of the more 

 common rock-making materials, showing their close similarity. The names 

 mica and feldspar each include several species : — 



Sili 



ca Quartz (p. 55). 



+ magnesia and water Talc (p. 61). 



" " Serpentine (p. 61). 



" -f lime or protoxyd of iron * Pyroxene (p. 60). 



" « " Hornblende (p. 59). 



" + alumina and protoxyd of iron Chlorite (p. 61). 



+ alumina Andalusite (p. 58). 



" Kyanite (p. 58). 



" -f fluorine Topaz (p. 59). 



" + oxyds of iron Staurotide (p. 58). 



" " " + potash or magnesia. Mica (p. 56). 



" -f lime and soda Scapolite (p. 58). 



" + lime, magnesia, iron, or manganese Garnet (p. 57). 



" + oxyd of iron Epidote (p. 57). 



