66 
AMERICAN GEOLOGY. 
able, in consequence of concealment by the soil, or by the adja¬ 
cent rocks. Fig. 5 illustrates this position. If a complete sec¬ 
tion of a hill in which a mass of granite cropped out, it might 
disclose granites of three periods, a, b, c (Fig. 2, p. 45), but 
if a portion only of the mass, a, could be seen, and a portion 
only of b, it would be difficult, if not impossible, to say whether 
the two masses were of the same age or not. Sir Charles Lyell 
has demonstrated that the oldest granites usually rest upon the 
i newer, and hence the term hypogene , the nether formed rock. 
The newer rocks may be connected with the surface by dykes 
or veins, as at a. In this case the masses of the rock with 
which they are connected, have cooled against the under side of 
a more ancient mass. Now that it is understood that cracks 
and fissures are formed in the rocks by cooling, it is ng longer 
difficult to explain how veins of granite, as well as the metal¬ 
liferous veins, are occasionally found in sedimentary as well as 
in those of igneous origin. Those rocks which repose upon an 
igneous mass, are more frequently traversed by veins than 
others which are superimposed upon them, proving that they are 
nearer the source whence all the fused materials originate. 
The age of granite, whether in veins or in dome-shaped masses, 
can be determined only approximately. If a mass of granite 
overlies the carboniferous rocks, it is certainly as new as those 
rocks, but it is possible it may be more recent than the trias, or 
new red sandstone. 
Fig. 3. 
Fig. 3 illustrates a small mass of granite, in Chester, Mass., 
where the bed is connected with two veins which penetrate 
through a fine mica slate. The bed itself was formed undoubtedly 
by an overflow of granite which passed through the vertical veins. 
This illustrates, on a small scale, the formation of some of the 
