LITHOLOGY, 147 
representing all the stages of alteration and decay, but that the original 
rocks were most diverse, both in their mineral constituents and in their 
structure. Rocks which to the eye appear substantially the same, are, 
after microscopic examination, found to be widely separated from one 
another. Closely adjoining dykes, which at first glance would be as- 
sumed to be identical, prove to be very different; and this makes it plain 
that we deal with a complicated question. We find that these rocks can- 
not be regarded as forming any defined system, but that they are proba- 
-bly eruptions that took place at intervals during those long past ages 
when our rocks were accumulated and elevated, and owe their great 
diversity to variations in the underlying melted matters, in the condi- 
tions of eruption that obtained place at different periods, and to altera- 
tions produced in them by subsequent ages. Any effort now to sub- 
-divide them, and to refer different classes to different times, and to make 
geological systems of them analagous to the Mesozoic system, would be 
nearly impossible ;—therefore we must take them as a whole, as an old 
mass of basic eruptive rocks, and treat them all together. 
Not merely in physical and chemical properties do these rocks differ 
from the later eruptions. The Mesozoic trap rocks form, as a rule, large 
and conspicuous dykes. The scenery of the lower Connecticut owes 
much of its beauty to their high, overhanging cliffs, for trap rocks usu- 
ally make impressive scenery. So it is with the European basalts. They 
commonly stand in conspicuous masses above the surrounding region; 
and many often visited places are dependent upon basaltic rocks for their 
celebrity. But in New Hampshire all this is reversed. The trap rocks 
cut through old crystalline rocks, which, being very hard, are not more 
rapidly denuded than are the trap rocks, and hence the latter are not 
brought into prominence. In fact, more often the trap rocks, on account 
of their basic composition, are more easily decomposed and disintegrated, 
and hence, when they are brought into prominence, it is commonly in 
an inverse way; for, by yielding more readily to wear and decay, their 
removal from their position in the crystalline rocks forms gorges or 
flumes, many of which are celebrated for their beauty. Our trap dykes 
are, moreover, very often of such small size that in no case would they 
make striking features in the landscape. 
The first person who directed his attention to the trap rocks of New 
