1877.] 



227 [Wadsworth. 



greenstone," and in his paper, as well as in the Danas', a good descrip- 

 tion of the decomposition of this rock, both in this and in other 

 localities, can be found, together with its formation of boulders in 

 situ. President Hitchcock mentions this rock in several places under 

 the head of greenstone in his final report on the Geology of Massa- 

 chusetts, however quoting mainly from Dr. Webster's paper. In the 

 paper of Mr. W. W. Dodge, 1 it is described as " ranging from feld- 

 spathetic to hornblende rock, often very micaceous, sometimes gran- 

 itic by the absence of hornblende." Mr. W. O. Crosby, in his 

 Report on the Geological Map of Massachusetts, (page 11) calls this 

 an " exotic diorite," and considers it probably of the Norian Age, 

 making it the same as his " Norian Rocks " of Nahant, Marblehead 

 and Salem. In the papers referred to, all agree in one thing, that, 

 in general, the rock is composed of feldspar and hornblende. 



The general appearance of the rock varies to quite an extent ; in 

 some places it is light brown or reddish from the predominance of 

 feldspar, in others dark green to quite black. It breaks up into irreg- 

 ular blocks, which by disintegration form rounded boulder-like 

 masses, and yield a reddish brown sand. All the other large dikes, 

 as well as part of the smaller trappean ones, disintegrate in the 

 same way. The resulting sand is composed chiefly of feldspar and 

 altered biotite or rubellan. Veins of reddish feldspar and of feld- 

 spar and hornblende extend through many parts of the rock. 



The stone from this and other localities of the same kind, has been 

 used to a considerable extent in Boston, Cambridge, Medford, Mai- 

 den, Somerville, Brighton, and other towns, for basement walls, un- 

 der pinning, steps, posts, fence walls, etc., and, when it is used above 

 ground it compares very favorably with the other building stones 

 used beside it, especially when hewn, a fact which is quite surprising 

 when we consider its ready disintegration in the parent bed. The 

 oldest monument of its use in Cambridge, that I know of, is the old 

 mile stone, now standing near Dane Hall, which bears date of 1734. 

 The front of the building now occupied by the Cambridge Gas Light 

 Company at Harvard Square is of this rock, and was erected in 1840. 

 The mass of the rock is too sombre to be used to any very great 

 extent in edifices except with lighter colored materials, and, further, 

 it would be well to make a most careful examination of it, where it 

 has been used, before it is either condemned or accepted as a durable 



i Proc. Boston Nat Hist. Soc, Vol. xvn, p. 415. 



