618 
ELEMENTS OF GEOLOGY. 
granites which are visible to man are older, on the whole, 
than the overlying or trappean formations. 
If different sets of fissures, originating simultaneously at 
different levels in the earth’s crust, and communicating, some 
of them with volcanic, others with heated plutonic masses, 
be filled with different metals, it will follow that those formed 
farthest from the surface will usually require the longest 
time before they can be exposed superficially. In order to 
bring them into view, or within reach of the miner, a greater 
amount of upheaval and denudation must take place in pro¬ 
portion as they have lain deeper when first formed and filled. 
A considerable series of geological revolutions must inter¬ 
vene before any part of the fissure which has been for ages 
in the proximity of the plutonic rock, so as to receive the 
gases discharged from it when it was cooling, can emerge 
into the atmosphere. But I need not enlarge on this sub¬ 
ject, as the reader will remember what was said in the 30th, 
32d, and 35th chapters, on the chronology of the volcanic and 
hypogene formations. 
