BASALTIC ROCKS. 
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Basaltic Rocks, 
The basaltic or trap rocks are not unfrequently 
met with in various parts of the United States. 
The word trap (step) is usually applied to rocks 
in which hornblende predominates. It was former- 
ly confined to basalt, properly so called. Some 
geologists now tell us that there is no real basalt 
in this country ; and Cleaveland remarks, that " the 
columnar and prismatic masses which exist in va- 
rious parts of the United States are a secondary 
basaltiform greenstone, which in some cases, per- 
haps, may be passing into basalt." 
Bakewell states, that " when hornblende and 
feldspar are intermixed and have a granitic struc- 
ture, they form what is generally called greenstone ; 
and if the hornblende and feldspar, or augite and 
feldspar, are intimately combined and finely granu- 
lar, they form basalt^ It should be recollected, 
then, that the greenstone is composed of distinct 
grains, or small crystals of feldspar and hornblende, 
so united as to give it a granular appearance, while 
basalt is a compact homogeneous mass of the same 
ingredients. Whenever this class of rocks assume 
a trap or columnar form, as in the East and West 
Rocks, near New-Haven, we shall find them to be 
fine-grained, and, of course, basalt. 
Greenstone trap occurs in the eastern and north- 
eastern parts of Massachusetts in rather extensive 
ranges, being the prevailing rock that encircles Bos- 
ton on the north, west, and south, after passing be- 
yond the graywacke and argillaceous slate that en- 
circle that city. In Charlestown particularly we 
often meet with beds of greenstone ; also at Rox- 
bury and Nahant, where it forms a vein 40 feet 
thick in argillaceous slate and syenite. In Weston, 
Waltham, Lincoln, Lexington, and West Cambridge, 
we find greenstone forming ridges, elevated some 
500 feet above the ocean. A ridge of greenstone 
