Chemical Effects of Greenstone. 



431 



the same relative position as above mentioned ; that is, the sandstone 

 is the superior rock. There it dips from 40° to 50° : but I\did not 

 perceive in it any cavities ; nor is the red color or the fissile charac- 

 ter destroyed. The Connecticut river here has worn away nearly 

 all the sandstone, except an occasional patch, for one or two miles : 

 but where these patches remain, a fine opportunity is afforded for ob- 

 serving the junction. And in some places I noticed that small 

 rounded masses of the amygdaloid were partially entangled in the 

 sandstone : as if, when the melted mass of greenstone was forcing 

 its way upward, and pressing hard against the incumbent sandstone, 

 portions of the former rock, while yet partially solidified, were worn 

 off and rounded by the latter. More frequently we see fragments of 

 the sandstone insulated in the greenstone ; being perhaps unmelted 

 portions of the former rock. 



For several feet below the surface of the amygdaloid at this local- 

 ity, it is not uncommon to see that rock divided into parallel portions, 

 whose surfaces correspond in dip and direction with the strata of 

 sandstone. The thickness of these layers is from one to four feet. 

 But they do not extend through the whole mass of rock, and can, 

 therefore, hardly be considered as genuine strata. 



The existence of so much amygdaloidal greenstone on the eastern 

 or upper side of the ridge, (for it must be recollected that such is the 

 dip of the sandstone embracing the trap as to render the eastern the 

 upper side,) while it is comparatively rare and far less porous and ir- 

 regularly situated in the lower parts of the range, leads naturally to 

 an inquiry as to the cause. It may be sufficient to say, that the gase- 

 ous matter extricated by the intense heat of a large mass of rock in a 

 state of fusion, would naturally be forced to its upper part by the 

 greater pressure below: although in the example described by Prof. 

 Silliman, such does not appear to have been the case. Since, howev- 

 er, the base of the rock on the east side of the ranges above named, is 

 more argillaceous and less crystalline than that of the rock on the 

 western or lower side, I am disposed to believe the former more fa- 

 vorable for retaining the gas or vapor than the latter. 



When the water is low we have an instructive exhibition of the 

 junction of greenstone with the subjacent sandstone at Titan's Pier in 

 South Hadley. A considerable part of the trap near the sandstone 

 is a breccia ; and one of the ingredients is clay, indurated almost to the 

 hardness, and exhibiting the light gray color, of hornstone, although 

 not exactly that substance ; especially if a mixture of limestone in the 



