Curvatures in Graywacke Slate. 



273 



ment made above, that the prevailing direction is easterly and wester- 

 ly, and the dip northerly, with the exception there named in Rhode 

 Island, and of a tract from Rhode Island to Randolph. Wheth- 

 er there is a particular line along which the strata change sud- 

 denly in their direction from north and south to east and west, or 

 whether the change is gradual, I have not been able to determine ; 

 though inclined to believe it sudden. The central parts of the tract, 

 near where the change must take place, is so covered with diluvium 

 as rarely to exhibit rocks in place. As to the cause of this anomaly, 

 I am disposed to believe that this graywacke belongs to two sys- 

 tems of elevation ; the one running nearly east and west, and the other 

 nearly northeast and southwest. In the conclusion of my Report I 

 shall examine this subject more particularly. 



It will be seen from the preceding statement of the direction and 

 dip of the strata, that there is much irregularity in the position of the 

 argillaceous slate connected with the graywacke ; particularly in 

 Charlestown. But this in general is easy to be explained, by the in- 

 trusion of masses of greenstone, or the proximity of sienite. 



The slaty structure of the slates included under graywacke, does 

 not always coincide with the stratified structure. I have observed 

 this to be the fact, particularly with a variety of the red quartz rock, 

 which in Randolph, Walpole, Wrentham, &c. become slaty, by tak- 

 ing into its composition a larger proportion of talc. 



In South Boston, and on Rainsford Islands, the argillaceous slate 

 contains a double set of seams, oblique to the strata seams ; and 

 thereby the rock is divided, often with great regularity, into tables 

 with rhombic or trapezoidal faces. (Nos. 360, 361.) 



In general, I doubt whether the argillaceous slate and the gray- 

 wacke slate of this formation, exhibit such striking tortuosities in 

 their layers, as European geologists describe in the corresponding 

 rocks in the eastern world. In some places, however, these curva- 

 tures are remarkable enough in New England. On Rainsford Is- 

 lands the argillaceous slate, although unusually fissile, is bent so as 

 to form a semicircle within the space of a very few inches. (No. 362.) 

 But in the southern part of Newport, Rhode Island, in the vicinity 

 of granite, we find the most remarkable curvatures in the graywacke 

 slate. 



The following sketch was taken from a cliff on the south east 

 shore of that town. It is from 15 to 20 feet high, and 30 or 40 long, 

 and the drawing does not at all exaggerate the tortuosities and irreg- 



35 



