Limestone. 



303 



of the vein. At present there is no seam at all in the slab, and ap- 

 parently it would not break more easily in one direction than in 

 another. As to the strip of darker limestone, d d, there is no more 

 difficulty in accounting for its presence, than for any other case of 

 close union between different varieties of a rock. But if we suppose 

 the two veins, a, a, to have been once united endwise, it is extremely 

 difficult to imagine how they could have been so slidden as to be 

 brought into their present condition. Dr. Macculloch has described 

 a similar case of disturbance in a slab of marble from Ireland in the 

 Transactions of the Geological Society.* But in that case it was not 

 difficult to imagine how the fragments of the vein, by a series of 

 slides, might have been displaced in the manner exhibited upon his 

 drawing. In the present case, however, I despair of being able to 

 explain that sort of double echellon movement, by which both the 

 vein and the dark mass of limestone have been displaced. 



I am not aware that in any case the limestone of Berkshire county 

 forms hills of any considerable altitude. In general mica slate, 

 with quartz rock, composes the peaks and ridges so striking and 

 sometimes so lofty, in the great valley of this county. Whether the 

 low level of the basset edges of the limestone results from the greater 

 liability of this rock to be worn away, or from its geological posi- 

 tion, I am not prepared to say ; though inclined to refer it to the lat- 

 ter cause. 



In general the limestone under consideration retains its characters 

 distinctly to the very line of junction with other rocks. But not un- 

 frequently the two rocks are intermixed near the place of contact. 

 The dark gray limestones, as already remarked, appear to contain 

 a mixture of argillaceous slate. Frequently we find scales of mica 

 disseminated in the limestones, and thin layers of talc. Where the 

 limestone comes in contact with mica slate, in Canaan, Ct. we find a 

 mixture in almost equal proportions, of carbonate of lime, mica, and 

 quartz. (No. 456.) 



In Stockbridge and the south part of Lee, two or three varieties of 

 limestone occur of rather a peculiar character. The most abundant 

 of these has externally a dark gray color, occasioned by one or two 

 foreign minerals, which do not decompose so rapidly as the carbon- 

 ate of lime. When the specimens are broken, the fresh surface is 



* Vol. 4. p. 393. 



