BASALTIC ROCKS. 



273 



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 whi<*.h exist in va- 

 rious parts of the United States are a secondary 

 basaltiform greenstone, w^hich 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 generRWy ^culled 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 



