Topography of Hornblende Slate. 



377 



has done. It certainly can lay claims to be considered only as a va- 

 riety of hornblende slate. 



I doubt not that other and still more compound varieties of horn- 

 blende slate might be found among our rocks : for this rock passes by 

 imperceptible gradations into almost all those with which it is associ- 

 ated. Under talcose slate, I have noticed, for example, a variety con- 

 taining hornblende, which might as well have been reckoned in this 

 place. Under mica slate, I have also noticed an amphibolic variety, 

 in which the hornblende sometimes predominates. 



Topography of the Hornblende Slate. 



Every deposit of hornblende slate which I have examined in New 

 England, is associated either with gneiss, talcose slate, mica slate, 

 or quartz rock. The small patch represented on the map in the vi- 

 cinity of the beds of limestone in Smithfield, R. I., is apparently as- 

 sociated with talco-chloritic slate ; though as we go westerly, we find 

 it alternating with gneiss : indeed, I suspect these three rocks are 

 sometimes interstratified. Much of the hornblende slate in the vicin- 

 ity of the lime quarries, is decided primitive greenstone, and green- 

 stone slate ; and it also passes insensibly into the talco-chloritic slate. 



Most decidedly associated with gneiss is the range of hornblende 

 slate represented as extending northeasterly from Grafton to Billerica. 

 In some places, as at Marlborough, its hornblende character is fully 

 developed, and it there becomes as nearly greenstone slate perhaps as 

 any rock in the State. But generally this stratum is more or less 

 intermixed with gneiss, so that sometimes one sees only traces of it. 

 Yet as I usually found the deposit well characterised in the region 

 where it is marked, I thought it proper to give it as a continuous 

 range on the map. Generally between this hornblende slate and the 

 granite on the east, the surface is composed of diluvium, so as to hide 

 the junction of the two rocks. 



Extending from Stafford in Connecticut, to Athol, in Worcester 

 county, the map exhibits another narrow stratum of hornblende slate, 

 very similarly situated to that just described. I have met with nar- 

 row beds of this rock still farther north, and probably it might with 

 truth be extended across the whole State. But I saw it so rarely 

 north of its present termination, that I thought it best not to carry it 

 across the State. This rock is generally crystalline in its aspect, and 

 is mixed with feldspar and quartz in considerable quantity : being 

 obviously in nearly all cases, but a variety of gneiss. And it ought 

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