Topography of Talcose Slate. 



357 



slate are shown on the map, separated by a range of granite. I am 

 not sure that the granite will be found extending uninterruptedly 

 through these towns as it is represented on the map. But in crossing 

 that region, I have always found one or two large beds of granite or 

 sienite ; and probably these rocks exist there in several beds in the 

 talcose slate and mica slate. Yet this alternation could not convenient- 

 ly be shown on the map. 



On the west side of the granite in Smithfield we find a peculiar 

 kind of mica slate, (No. 675,) and a talcose slate, which has been al- 

 ready described as a whetstone slate. The extent of surface occupied 

 by this variety—which is well characterised talcose slate — I am un- 

 able definitely to state ; though it cannot be but a few miles in any 

 direction unless it be towards the southwest. 



In tne east part of Smithfield, and on the east side of the gran- 

 ite range or bed, the slate is often distinctly talcose ; especially in the 

 vicinity of the soapstone quarry, near the village called Maysville. 

 But as we cross the strata towards the southeast, the characters be- 

 come very obscure and perplexing. In one place the rock can hard- 

 ly be distinguished from argillaceous slate ; in another it greatly re- 

 sembles graywacke slate ; in another it is distinct chlorite slate ; and 

 in another it becomes hornblende slate. Epidote too forms an ingre- 

 dient in a large proportion of the rock. In short, this series of slaty 

 rocks is one of the most perplexing with which the geologist meets. 

 It extends over nearly the whole of Cumberland, where it is inter- 

 stratified with quartz rock, and is succeeded on the east by quartz 

 rock and graywacke. The strata of all these rocks run nearly north- 

 east and southwest, and dip to the southeast. Taken as a whole, I am 

 inclined to denominate the predominant part of the series talco-chlorit- 

 ic slate. Nor can I resist the impression, that the whole series is 

 the graywacke slate, which, by the agency of heat, has been partially 

 converted into argillaceous slate, talcose slate, chlorite slate, and 

 hornblende slate ; the heat not having been powerful enough com- 

 pletely to accomplish the transmutation. And the contiguity of 

 granite and sienite would furnish the heat which this hypothesis 

 demands. 



But whatever may have been the origin of this series of rocks, a 

 perusal of Von Oeynhausen and Von Dechen's paper on the junction 

 of the granite and killas rocks of Cornwall, in England, has led me to 

 the opinion, that they greatly resemble the killas of that country.* 



* Philosophical Magazine, Vol. 5, N. Y. p. 161. 



