40 



erous formation of Plymouth and Bristol Counties would 

 appear to be an exception to the above statement ; but it 

 should be said of this area that it is largely covered by drift 

 material, especially toward the southern border, and compara- 

 tively few observations of the rocks have been made. Slates 

 are said to occur in Abington ; and it is known that there are 

 occasional small masses of diorite included in or cutting 

 through the granite. These appear to be very irregular in 

 outline, and are most numerous toward the west, where we 

 approach the large area colored as diorite on the map. The 

 accurate mapping of these limited patches is practically im- 

 possible ; and, since we know, from such observations as have 

 been made, that the diorites can form but a small proportion 

 of the whole, I have ignored them on the map, marking the 

 entire region as granitic. A more detailed map, such as it is 

 hoped the student of the near future will construct, would show 

 the boundaries of the granitic areas to be much more complex 

 than they are here represented, where we have hardly an 

 approximation to the intricacy of nature. 



A farther inspection of the map will make it evident, how- 

 ever, that the granite is not without some system in its dis- 

 tribution. The granite areas north of the Boston and Albany 

 Railroad, and south of the Essex branch of the Eastern Rail- 

 road, are arranged along a line having a direction about 

 N. 50° E. ; forming a discontinuous band of variable width, 

 which extends from Natick to Rockport, on Cape Ann, a dis- 

 tance of forty-five miles ; and we readily find in this wall of 

 granite, probably the most resistant rock of this region, a suffi- 

 cient cause for the existence of this prominent headland. The 

 slate and conglomerate in Newton and Watertown are proba- 

 bly underlaid by granite, which would account for the discon- 

 tinuity of the range here. The granite in Marblehead and 

 Swampscott, though considerably out of the line of this belt, 

 is evidently connected with it : and, if the water were re- 

 moved from those shores, we should probably find that these 

 isolated patches are really united, forming an irregular 



