92 
IDDINGS. 
“ Our knowledge, indeed, of the laws which determine the 
formation of different minerals from their elementary in¬ 
gredients is at present too limited for the solution of such a 
question.” 
Haying admitted his lack of knowledge on this subject, he 
proceeds to construct a very remarkable hypothesis, by which 
he expects to obtain trachyte and basalt from a granite, and 
which is here quoted in full, because he refers to it in a sub¬ 
sequent edition of his work on volcanoes. 
“It would not, indeed, be difficult,” he says, “to conceive 
the production of ordinary trachyte, by a very slight change, . 
from a granitic original; for the process of intumescence, 
when carried far, may easily be supposed to change the 
feldspar crystals from compact to glassy; to dissolve the 
whole or the greater part of the quartz in the aqueous 
vehicle, forcing it to assume the crystalline form of feldspar 
on consolidation (as in graphic granite); finally, to volatilize 
the mica, of which part would recrystallize in more perfect 
crystals, either of mica or augite, on cooling; and a part, per¬ 
haps, give rise to specular or magnetic iron disseminated 
through the rock or lining its pores and fissures. 
“ In like manner we may imagine the production of basalt 
to have been caused by the exposure within the vent of a 
volcano of an intumescent mass of granite to reconsolida¬ 
tion, effected by the augmentation of temperature and con¬ 
sequent expansion of its lower beds. In these, parts the 
extreme heat may be supposed to volatilize the mica and 
other ferruginous minerals, while the intense pressure would 
separate them in a gaseous state from the feldspar, thus leav¬ 
ing a feldspathose lava, with very little iron, in one part of 
the chimney and occasioning the crystallization of a highly 
ferruginous lava in another. 
“ The subsequent intumescence and protrusion of these 
lavas might produce alternate currents of trachyte, clink¬ 
stone, or compact feldspar, and basalt or graystone.” 
With reference to the occurrence of trachyte and basalt, 
Scrope maintained, in opposition to Humboldt and Beudant, 
