406 



Scientific Geology. 



are very coarse, and the feldspar, which is foliated, is of so dark a 

 color as with difficulty to be distinguished from the hornblende. (Nos. 

 1127, 1128.) The feldspar is arranged in stripes, like a ribbon, as 

 in the sienite near Boston. 



The compound that has now been described, constitutes the princi- 

 pal part of the greenstone dikes, ridges, and hummocks, in Massa- 

 chusetts. (Nos. 1106 to 1135.) The same compound occurs also 

 in other forms J as will be seen in the sequel. 



2. Columnar. This differs from the preceding variety only in 

 form : for its composition is almost uniformly the same. Nearly all 

 the greenstone in the valley of the Connecticut exhibits more or less 

 of a conatus at a columnar structure, except the tufaceous variety. 

 Yet it is the finely granular variety, that exhibits the most perfect 

 forms. A similar conatus appears in some of the beds of green- 

 stone in the eastern part of the State, especially in Charlestown : but 

 these columnar masses are so imperfect, compared with some of 

 those in the Connecticut valley, that I shall limit my remarks en- 

 tirely to the greenstone occurring in that valley: and as it is more 

 convenient, I shall in this place give the topography of the variety 

 under consideration. 



Nearly all the ridges of greenstone in the valley of the Connecti- 

 cut, (for a reason that will appear in the sequel,) present on their 

 western sides, a nearly perpendicular face. Usually, however, the 

 angular fragments that have fallen from these precipices, have accu- 

 mulated at the bottom so as to form a steep talus, reaching half or 

 two thirds the distance to the summit ; and sometimes entirely to the 

 top. Where the rock appears in the face of the cliff) it is almost al- 

 ways more or less columnar ; sometimes as much as 30 or 40 feet in 

 height. In some cases one set of columns is separated from another 

 set, above or below, by a stratum or mass of trap tuff 



There are, however, only a few places where these columns are 

 very perfect. Along the west side of the greenstone ridge that forms 

 the eastern part of Deerfield mountain, in several places, about a 

 mile east of the village, they exhibit great regularity. Usually their 

 diameter here, — and the same remark will apply to every other lo- 

 cality, — is between two and three feet ; rarely as small as one foot ; 

 and the number of their sides between four and six. They are some- 

 times distinctly articulated ; the joints varying from one to three or 

 four feet in height. The articulations are usually curved, at their ends 

 presenting frequently a convexity on the upper side, and a concavity on 



