276 



BASALTIC ROCKS. 



about 100 miles above Hallowell, on the Kennebeck 

 River, forming the summits 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 well-denned ; 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. u 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."f 



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, 

 f Parker's " Exploring Tour beyond the Rocky Mountains," 

 p . 220. 



