268 



Scientific Geology. 



ceous slate, or quartz rock, and contains encrinites ; so that probably 

 its age corresponds with that of graywacke. But as it is entirely 

 disconnected with the graywacke, I shall defer a description of it till 

 I speak of the limestones. 



Topography of the Graywacke. 



I have anticipated so much of this head, that brevity may now be 

 consulted. It will be seen by the map that this formation is confined 

 exclusively to the eastern part of Massachusetts and Rhode Island ; 

 and that it exists in several detached patches. On the second, or the 

 present edition of the geological map, I have connected the strip of gray- 

 wacke passing through Dedham, Walpole and Wrentham, with the 

 broad deposite south of the last named place. Nor should I be sur- 

 prised, if future observers should discover a connection between the 

 graywacke range in Dedham, Canton, and Randolph, and that in 

 Quincy, Dorchester, &c. ; although I failed in finding it. The fact 

 is, this rock in no place rises into any thing like mountain ridges ; and 

 for the most part, it occupies extensive plains, or gently undulating 

 ground. Diluvium, also, is extremely abundant over almost every part 

 of it; so that it is only occasionally, and often at distant intervals, that 

 graywacke is seen in place. This is particularly the case in the most 

 extensive tract of the graywacke, which embraces the greater part of 

 Rhode Island, with nearly every other island in Narraganset Bay, 

 and a strip of uncertain width on the west shore of that bay, as well 

 as a narrow tract on the east shore in Little Compton ; and as it ex- 

 tends northerly into Massachusetts, occupies the surface of nearly 

 twenty towns in Bristol and Plymouth counties. In Swansey and 

 Somerset, the most abundant conglomerate of this formation, (which 

 for the sake of distinction I shall call the Roxbury conglomerate, be- 

 cause in that place its characters are strongly developed,) forms 

 several hills of one or two hundred feet in height, producing striking 

 outliers in the landscape. In Dorchester, Roxbury, Newton, Brook- 

 line, and Brighton, the hills of the same rock are of moderate eleva- 

 tion ; rarely exceeding 200 feet : yet this is the most hilly part of the 

 graywacke formation in Massachusetts. Now its low level and the 

 abundance of transported fragments that overspread it, render it ex- 

 tremely difficult to ascertain its limits. 



On the first edition of the map, I put down a patch of graywacke 

 in Duxbury. But on more mature reflection, I feel satisfied that I 

 have not evidence enough, that the slaty epidotic rock which occurs 



