46 
AMERICAN GEOLOGY. 
classes. Fig. 2 illustrates the mode of formation of the pyro-> 
crystalline rocks, and the relative age of each mass. 
§ 39. The age of rocks deduced from the 'perfection of their 
crystalline state. Assuming the former fluid condition of the 
earth by heat, and its present condition by the loss of it, it may 
be inferred that the greater intensity of heat produced fusions 
far more perfectly in the earlier than in the later periods. We 
may conceive, then, that the products of a perfect will differ 
in structure from those of an imperfect fusion. The former 
will be longer in cooling, and the particles of the mass will be 
in a more favorable condition to move freely and arrange 
themselves according to their respective affinities. The first 
products arising from the cooling of the earth’s crust are the 
granites. These rocks are preeminently crystalline, and their 
j perfect crystalline condition resulted from the former high 
temperature to which their masses v^ere subjected when the 
whole earth was in a molten state. The products of the subse¬ 
quent periods, when the earth’s crust had materially cooled, 
are less crystalline. Thus granites in mass, the first products 
of cooling, are traversed by granitic veins which constitute a 
second stage in granitic productions. In periods still later trap 
dykes appear, and we find in their structure indications of a 
diminished fluidity by fusion: they are black granular or com¬ 
pact and homogeneous, or imperfect crystals of feldspar pervade 
the mass. The traps proper traverse the earlier and later rocks, 
and cut the more ancient granitic veins, but are themselves 
more rarely intersected by granitic veins; and even the massive 
greenstones are rarely, if ever, intersected by granitic veins. 
Granitic veins and beds, however, are products of all periods as 
late at least as the chalk; but their frequency is diminished 
in the ratio of a thousand to one, comparing the granitic with 
the cretaceous periods. 
The foregoing considerations seem to favor the doctrine that 
the imperfect crystalline structure of the later igneous products 
is due in part to a diminished heat in the earth’s crust. We 
have never seen a granitic vein intersect a trap dyke or a mass 
