554 
ELEMENTS OF GEOLOGY. 
Fig. 607. Fig. 60S. 
Graphic granite. 
Fig. 607. Section parallel to the laminae.—Fig. 608. Section transverse 
to the laminae. 
ally occurs in large crystals of a white or flesh color; and 
2 dly 5 feldspar in smaller crystals, in which soda predomi¬ 
nates, usually of a dead white or spotted, and striated like 
alhite, hut not the same in composition.* 
As a general rule, quartz, in a compact or amorphous state, 
forms a vitreous mass, serving as the base in which feldspar 
and mica have crystallized ; for although these minerals are 
much more fusible than silex, they have often imprinted their 
shapes upon the quartz. This fact, apparently so paradoxi¬ 
cal, has given rise to much ingenious speculation. We should 
naturally have anticipated that, during the cooling of the 
mass, the flinty portion would be the first to consolidate; 
and that the different varieties of feldspar, as well as garnets 
and tourmalines, being more easily liquefied by heat, would 
be the last. Precisely the reverse has taken place in the 
passage of most granite aggregates from a fluid to a solid 
state, crystals of the more fusible minerals being found en¬ 
veloped in hard, transparent, glassy quartz, which has often 
taken very faithful casts of each, so as to preserve even the 
microscopically minute striations on the surface of prisms of 
tourmaline. Various explanations of this phenomenon have 
been proposed by MM. de Beaumont, Fournet, and Durocher. 
They refer to M. Gaudin’s experiments on the fusion of 
quartz, which show that silex, as it cools, has the property of 
remaining in a viscous state, whereas alumina never does. 
This ^‘gelatinous flint” is supposed to retain a considerable 
degree of plasticity long* after the granitic mixture has ac¬ 
quired a low temperature. Occasionally we find the quartz 
and feldspar mutually imprinting their forms on each other, 
affording evidence of the simultaneous crystallization of 
both.f 
* Delesse, Ann. des Mines, 1852, t. iii., p. 409, and 1848, t. xiii., p. 675. 
t Bulletin, 2e serie, iv., 1304; and D’Archiac, Hist, des Progres de la 
Geol., i., 38. 
