276 
BASALTIC ROCKS. 
about 100 miles above Hallo well, on the Kennebeek 
River, forming the smiimits of several mountains 
from 200 to 300 feet high. Here also it has a 
columnar structure, as at Mount Holyoke, near 
Northampton, and at Deerfield. The prisms pre-* 
sent from three to six sides, their edges being 
straight and w^ell-dehned ; and their general aspect 
is that of bricks standing on their ends. 
Greenstone abounds also among the Rocky 
Mountains, where it is seen forming immense col- 
umns of a pentagonal form. " On examining the 
bluffs, or perpendicular banks of rivers and mount- 
ains," says Mr. Parker, " I have numbered from 
between ten and twenty different strata of amyg- 
daloid, basalt, and breccia. These appear to be 
thrown up through dikes, or through craters rising 
in different succession one above another. In some 
places, the lowest formation was pudding-stone, 
above this amygdaloid, then a stratum of angular 
fragments of basalt and amygdaloid, and some- 
times intermixed with lava,* which may be called 
breccia ; and over these basalt, frequently in regu- 
lar pentagons, which vary in size from one to five 
feet in diameter, and in regular articulated sections ; 
and upon the basalt another stratum of breccia; 
and again upon these is superimposed another 
stratum of basalt, or in some cases amygdaloid; 
and, in the same manner, strata above strata, in some 
places to twenty in number. These strata vary in 
depth from a very few feet to thirty or forty ; and 
the whole series rises from 200 to 500 feet."t 
The accompanying plate, representing a ledge of 
greenstone rocks on the Columbia River, will con- 
vey a good idea of these formations. The channel 
of the Columbia in many parts is walled up on its 
sides, and studded with islands of basaltic rocks, 
* Probably trap tufa, such as we have already described, 
t Parker's '* Exploring Tour beyond the Rocky Mountains," 
p, 22G. 
