LITHOLOGY, I47 



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 



