THE USES OF FOSSILS 403 
Age of Igneous and Metamorphic Rocks. — Many of the meta- 
morphic rocks exist below the oldest fossil-bearing strata, and are 
hence -classed in the Archean; but there are others of ages 
‘later than this. If they contain fossils, their age can be deter- 
mined; but as fossils are commonly destroyed by the processes 
which have brought about the metamorphism, this means of 
classifying them is seldom available. In such cases it is often 
impossible to tell their age, though sometimes their relation to 
neighboring fossil-bearing rocks furnishes the key. In the ma- 
jority of cases, however, these rocks are of doubtful age, though 
as careful studies continue, more and more of them are being 
removed from the doubtful list. 
Igneous rocks contain no fossils, and hence their age is very 
often doubtful. Still, at times, their relation to other rocks gives 
their approximate age. For instance, in the Connecticut valley, 
near Meriden, Connecticut, there are lava flows buried in sand- 
stones, the fossils of this enclosing rock clearly pointing to the 
Triassic period. Again, in New Jersey, there are intrusions of 
the same igneous rock cutting the same kinds of sandstone, and 
hence of later date than these strata through which they pass; 
but upon the Triassic beds, rest later rocks of the Cretaceous 
age, and these are nowhere cut by the dikes. Hence we conclude 
that they came to their place at some time between the Triassic 
and the Cretaceous periods. 
