288 Scientific Geology. 



Topography of the Argillaceous Slate. 



With the exception of that in the vicinity of Boston, the map ex- 

 hibits hut three ranges of argillaceous slate ; viz. in the counties of 

 Worcester, Franklin, and Berkshire. And it happens that in all 

 these cases, except perhaps the first, the principal part of the range 

 lies out of the state, either in New Hampshire, Vermont, or New 

 York. Two miles south of the center of Halifax, Plymouth County, 

 also, I found a delicate variety of argillaceous slate, which I was 

 informed was discovered in digging wells, and that it lay immediately 

 upon granite. (No. 363.) But whether it exists to any considerable 

 extent in that region, I am unable to say. 



Worcester County Argillaceous Slate. 



Some geologists would probably regard the slate that forms the 

 roof and floor of the mine of anthracite in Worcester, as argillace- 

 ous slate ; and maintain that the range of this slate in Worcester 

 County, extends at least as far south as that spot. But I regard that 

 slate rather as a fine mica slate, much impregnated with carbon, 

 which gives it the appearance of argillaceous slate. In almost every 

 case the scales of mica are quite distinct : and at a short distance from 

 the mine, the rock assumes the characters of mica slate distinctly ; 

 though here, as in most of the range of mica slate extending from 

 the mouth of Merrimack to the State of Connecticut, much of the 

 rock is so quartzose that it might perhaps be regarded as quartz rock. 

 I have not found any well characterized argillaceous slate, south of 

 Boylston. And north of this place, the country is so much covered 

 with diluvium, and so little hilly, that the slate does not often come 

 into view. I found the range, however, to become narrower on ap- 

 proaching the north line of the State. Its characters appear most 

 fully developed in Lancaster, where it has been quarried for roofing 

 slate ; and here the range is broadest. How far it extends into New 

 Hampshire I have not ascertained ; though I should not be surprised 

 to find it even crossing that state. In passing from Groton to Town- 

 send, I saw frequent examples of protruding masses and veins of 

 granite in this slate. It passes on either side into the peculiar mica 

 slate, already spoken of in Worcester County ; and in this latter rock 

 protrusions of granite are not unirequent. 



