Varieties of Greenstone. 



411 



With abase of hornblende and feldspar: the first variety of green- 

 stone above described. I have never seen any of this amygdaloid 

 very regularly columnar : still it very frequently exhibits a columnar 

 tendency. And it is a curious fact that the cavities often run length- 

 wise of the column, and are parallel to one another ; so that the rock 

 resembles a block of wood, which worms have bored through repeat- 

 edly in a logitudinal direction. I have observed some of these cavi- 

 ties a foot or two in length. (No. 1166.) On account of the com- 

 pactness of this rock, these cavities are usually filled with foreign sub- 

 stances, such as calcareous spar, chalcedony, quartz, chabasie, Lin- 

 colnite, &c. The best spot that I know of for obtaining specimens of 

 this rock, is one mile directly east of the academy in Deerfield. 



In the same ledge, as well as in other ledges, the amygdaloid 

 abounds in spherical or spheroidal cavities, filled with quartz or cal- 

 careous spar. 



What particular causes produced these different forms in the cavi- 

 ties, it may not in the present state of knowledge be possible satisfac- 

 torily to ascertain. That they were all produced by an elastic fluid, 

 while yet the rock was in a plastic state, seems now generally admit- 

 ted. Must not the different forms which they have assumed, be im- 

 puted to inequality of pressure? And yet the air vescicles in a mass 

 of ice exhibit the same variety of shapes, some of them being cylin- 

 drical, some spheroidal, and some spherical: nor can we in this case 

 impute their form to inequality of pressure. But whatever the cause 

 be, as in both instances the effect results from refrigeration, may it not 

 be the same ? 



The most usual amygdaloid in the Connecticut valley has a base 

 which appears to be wacke. It occupies, as already remarked, the 

 easterly part of the ridges wherever I have examined them. For 

 the most part, it is liable to partial decomposition to a considerable 

 depth from the surface, and the imbedded minerals have entirely dis- 

 appeared. When they still remain, calcareous spar is the most com- 

 mon. Not unfrequently, however, foliated chlorite occupies most of 

 the cavities ; and sometimes, as at Turner's Falls, they are filled with 

 chlorophoeite. Green earth, or earthy chlorite, is still more frequent- 

 ly present. Sometimes the base is of a reddish hue ; but commonly 

 of an earthy gray. In such cases the rock exceedingly resembles a 

 toad in appearance, and is probably similar to, if not identical with the 

 loadstone of some of the English geologists. When the cavities are 



