Varieties of Mica Slate. 



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In Norwich and Enfield this variety has been extensively employed 

 for whetstones : the former locality is far the best, and the latter is 

 now nearly or quite abandoned. 



In general this variety occupies the highest place in the mica slate 

 series. Thus we find it on both sides of the Valley of the Connec- 

 ticut, when first we pass on either side of the river from the new red 

 sandstone ; and the whole of the mica slate formation in Worcester 

 County is of this description. 



This variety is very nearly allied to quartz rock. Indeed, in re- 

 spect to extensive tracts, it is often difficult to say whether it should 

 be denominated quartz rock or mica slate. Sometimes it exhibits a 

 double set of strata seams j one set being oblique to the other. It 

 contains also not unfrequently beds or tuberculous masses of white or 

 sometimes blood red quartz. 



9. Anthracitous Mica Slate. This is simply a very fine grained 

 mica slate, approximating to clay slate, which has been impregnated 

 and rendered black and shining by carbon. I am disposed to re- 

 gard the rock constituting the immediate roof and floor of the an- 

 thracite bed in Worcester, as belonging to this variety, although 

 I am aware that it has been generally regarded as argillaceous 

 slate. But I think that in all cases careful examination will detect 

 the mica. Of this, however, more in another place. This variety 

 occurs, also, in Ward and in Dudley. (Nos. 717 to 719.) 



10. Plumbaginous Mica Slate. This rock differs from the last 

 only in exhibiting the gray aspect of plumbago, rather than the dark 

 color of anthracite. But probably in most cases very little plum- 

 bago is present. Yet the resemblance is often striking. This 

 variety occurs frequently among the newer beds of mica slate ; 

 as for instance, on the east side of Connecticut river, in Southamp- 

 ton, Conway, Shelburne, &c. (Nos. 713 to 715 and 718 and 719.) 



1 1. Conglomerated Mica Slate. In Haverhill and Amesbury I ob- 



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