212 



quartz. The exotic diorite can be traced east half a mile or 

 more. 



I consider these beds as representing the north side of the 

 Black's Creek and Rock Island Cove anticlinal, and to my mind 

 it is clear, that in this part of the Boston basin the structure of 

 the conglomerate and slate is not determined by the outlines of 

 the granite masses, but the faults which appear usually to bound 

 the latter have cut through the folds of the Primordial beds 

 irregularly and without essentially disturbing them. This is 

 well illustrated by the border of the granite as it zigzags across 

 the strike of the slate from Hayward's Creek north-westerly ; so 

 that the anticlinal in question, which is complete and symmetrical 

 on the east, is more than half cut away on the west. One and 

 three-fourths miles W. by S. from the section on Randolph 

 Turnpike, exactly the same rocks crop again, on the west side 

 of Robbins Street, between Washington Street and Pine Tree 

 Brook. There are no other exposures of uncrystallines on this 

 line east of the Boston and Providence Railroad. 



The country between this long anticlinal skirting the Blue 

 Hills, and the Neponset River, an area of at least ten square 

 miles, is almost wholly drift-covered, and the structure is mainly 

 a matter of conjecture. This tract widens rapidly eastward ; and 

 the strike of the rocks along the north side appears to shift 

 toward the north-east, so that it is parallel with the general 

 course of the Neponset. The hypothetical nature of this part of 

 the map will be appreciated, when it is stated that the large and 

 eastwardly expanding area marked as slate includes but one 

 rock exposure, besides those already described : this is at the 

 slate quarries north of Sachem's. Creek, in North Quincy. 



The conglomerate on the south shore of Squantum, and south 

 of the railroad atMiltonLower Mills, dips S. 20°-30° E. steeply, 

 60°— 80° ; with indications, especially at the last-named locality, 

 of a passage to slate southerly. Between this range of con- 

 glomerate and the Black's Creek and Rock Island Cove anticli- 

 nal I think there are at least two synclinals of slate. These 

 folds, like the area in which they are included, probably expand 



