106 
AMERICAN GEOLOGY. 
but still they are associated with gneiss and hornblende in the 
vicinity. A long list of localities might be made out of the 
kinds of rock prevailing at different points in the Union; but 
it is impossible to show, at the present time, that there exist 
important relations between any two distant points. At the 
most distant points of our country these rocks preserve a great 
similarity of structure and of character, which renders it impos¬ 
sible to recognize by specimen the part of the country they 
represent. They are traversed also by granitic veins, the com¬ 
position of which exhibits everywhere the same variations. 
There are districts, however, in which trap is much more 
abundant than in others; and these districts furnish us with a 
greater remove from the common character of the country at 
large. I shall speak of the peculiar rocks of certain districts 
when I have occasion to take up the subject of mines and 
mining. 
PYROPLASTIC ROCKS. 
§ 78. The first section embraces those rocks which are sup¬ 
posed to have been erupted through fissures in a molten state, 
and to have cooled beneath the ocean, and hence I have deno¬ 
minated them submarine. The greenstones amygdaloids, ba¬ 
salts, traps and porphyries, are comprehended in the section. 
The second section embraces those which have been erupted 
from craters, and have cooled in the open air, and hence I have 
called them subcerial. They embrace the modern lavas, of all 
kinds: the vesicular lava, obsidian, volcanic arks, &c. 
1. Sub7narine .—The rocks of this section are lithologically 
the same as in all parts of the globe. A traveler who speaks 
of trap in the greenstone or basalts of Australia, is understood 
by us in America. The greenstones of the Hudson river scarcely 
differ from those of Connecticut or Nova Scotia. Greenstone 
is massive, vesicular, columnar, and porphyritic. The first is a 
heavy black or grayish black rock, either occupying fissures in 
other rocks, or lying upon them, having been forced out from 
beneath in a molten state, and in a condition to overflow the 
region adjacent to the fissures. The term trap seems to be re- 
