ELEMENTS OF GRANITE. 
47 
of greenstone, although some granitic veins are newer than 
many veins and masses of trap. 
The evidence of age, however, when deduced from structure 
alone can not be relied upon, only so far as it indicates a 
general diminution of temperature; traps and greenstones never 
forming those parts of the earth’s crust which belong to the 
most ancient periods—'the rocks of the most ancient periods 
being represented by granites and gneiss, whose structures are 
eminently crystalline. 
Metallic veins—those of iron in northern New York, the 
auriferous quartz veins of Virginia and North Carolina—are 
traversed by dykes of trap or greenstone, and hence the former 
are older than the latter; and we have never seen the former 
traversing the latter, though in Derbyshire, England, metallic 
veins pass into them. Taking a general view of facts, how¬ 
ever, as they are developed upon a large scale, we are inclined 
to adopt the opinion, that the prevalence of the intersecting 
dykes of trap are due to general and not to local causes, and 
that this cause will be found connected with the cooling of the 
earth’s crust. 
§ 40. Special considerations respecting the elements of granite 
and its allied rocks. Feldspar is the predominating element in 
all the massive pyrocrystalline rocks. It is a fusible compound, 
but the mass of rock in which it is so abundant may have been 
more fusible than feldspar by itself. The three principal kinds 
of feldspar, all of which are quite common in American granites, 
are composed of 
Prismatic Feldspar. 
Albile. 
Labradonite. 
Silex, 
65-40 
70-7 
53-70 
Alumina, 
18-60 
19.8 
29-90 
Lime, 
00-20 
0-3 
12-00 
Potash, 
15*70 
0*0 
0-90 
Soda, 
o-io 
9-0 
4-50 
The presence of the alkalies and alkaline earths, while they 
promote as fluxes the fusion of the mass, materially contribute 
to its disintegration and decomposition. In consequence of 
