Columnar Greenstone. 



407 



the lower. The breadth of the sides is considerably unequal ; and 

 with this exception, perhaps, these columns might compare in reg- 

 ularity with those of basalt from Ireland. 



I have already given a general description, in the second part of 

 my Report, of an exhibition of greenstone columns towards the 

 southern extremity of Holyoke, in the western face of the hill. At 

 that place we see but little of articulation : but a most remarkable 

 disintegration, or rather exfoliation, is there constantly going for- 

 ward ; as the immense number of fragments at the base of the cliff 

 testifies. The pieces that split or scale off, are of almost every shape ; 

 but they are commonly rather thin, sometimes in curved laminae an 

 inch or two thick. When the lower part of a column begins thus to 

 scale off, the fissures take such a direction as to leave the under side 

 of the column, still projecting from the precipice, in the form of a 

 hemisphere, or more commonly in the form of paraboloid ; and not 

 unfrequently of a lenticular form. And in one spot at least, the 

 upper portion of two or three rows of columns is suspended 

 over the head of the observer, appearing like numerous iron kettles, 

 not less than three feet in diameter, hanging from the rock. This is 

 certainly one of the most singular natural objects that I have ever 

 met with ; nor can one feel at perfect ease beneath such a piazza, 

 when he sees by how feeble a hold these masses of immense weight 

 are sustained ; and how instantly one of them falling would grind 

 him to powder. 



I have been at a loss to decide, whether the exfoliation which is 

 exhibited at this spot, takes place according to an original structure 

 of the rock, or is produced by the natural action of the disintegra- 

 ting agents ; such as air, moisture, heat, and cold, upon rocks of a 

 peculiar form. I can hardly admit the latter supposition ; when, on 

 breaking the fragments, they are found, except for a mere line at the 

 surface, to be so entirely unchanged. Yet this curved form of exfo- 

 liation is not the only one exhibited on this greenstone range. More 

 frequently the columns split longitudinally, into somewhat irregular 

 pieces, from one to six or more inches in diameter. All along the 

 western side of Mount Tom, examples of this kind may be seen ; 

 and the quantity of fragments of this sort, accumulated at the base of 

 this mountain, is immense. Sometimes these fragments are very 

 regular in their forms ; producing prisms of three, four, five, &c. 

 sides, and three to six inches in diameter. (Nos. 1 1 36, 1 137, 1 138.) 

 Again, as at Titan's Pier, described in the second part of my Report, 



