270 Scientific Geology. 



composed of the rock which is thus developed. On this principle, 

 the geological character of the principal islands in this harbor may 

 be set down as follows : 



Noddle's 



Castle 



Thompson's 



Spectacle Islands 



Long Island 



Pedock's 



Gallop's 



George's 



Level's 



Deer Island 



Apple 



Great Brewster 



Moon Island Conglomerate 

 Hangman's Island Sienite 



Rainsford Islands 



Middle & Outward Brewster 



Boston Light 



Egg Rocks 



Calf Island 



Green's Island 



Governor's Island 



3 



I 



CD 

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GO 



It will be seen that argillaceous slate is the predominating rock on 

 the outermost of these islands. In general it is quite hard, and has 

 so little of a slaty structure, that one might well hesitate to call it ar- 

 gillaceous slate. Frequently it appears to be a coarse variety of no- 

 vaculite. Argillaceous slate likewise appears on the southern side 

 of the promontory of Hull ; although the surface is for the most part 

 diluvial. 



There can be little doubt that the peninsula of Boston has a foun- 

 dation of argillaceous slate. This is, indeed, the only rock that has 

 ever been found there in place. And from the occurrence of argil- 

 laceous slate in South Boston, and in Charlestown, with a northerly 

 dip in both places, it would be very surprising if any other rock 

 should be found in Boston ; unless it were an intruding mass of trap 

 rock. But the slate on the peninsula is buried deep by clay, gravel, 

 and sand ; although, from the quantity of diluvium found there above 

 the tertiary beds, I have been led to color the peninsula as a diluvial 

 deposite. 



The only remaining tract of graywacke to be noticed, is one of 

 limited extent, along Parker River in the south part of Newbury, and 

 extending I believe into Rowley. It consists of gray red and varie- 

 gated slates, slaty compact feldspar, with talc, and a conglomerate 

 resembling that in Roxbury. Red compact feldspar lies between 

 this rock and the sienite ; and some of the nodules of the conglom- 

 erate consist of the red compact feldspar. This tract of the gray- 

 wacke appears to be very interesting, and deserves a more thorough 

 examination. 



