265 



length along the east bank of Weymouth Back River, from 

 8()() to 1,300 feet south of Beal's Cove, are three dikes from 

 f) to 10 feet or more in width and trending- approximately east- 

 west. 



AGE OF THE IIINGIIAM STRATA. 



The principal facts bearing upon this problem have been 

 presented in the preceding pages, and it remains now simply 

 to marshal the scanty evidence and note its collective value. 

 Paleontological evidence is, at present, wholly wanting ; 

 although we may reasonably entertain the hope that fossils 

 will yet be found in the slates or sandstones of Hingham. The 

 lithological evidence, although it might be said to point to 

 the correlation of the Hingham slates with those of Weymouth 

 and Braintree, is certainly very unreliable in a case like this ; 

 and, furthermore, it is entirely at variance with the plain 

 indications of stratigraphy. But the stratigraphic evidence, 

 again, is far from direct or satisfactory, since the deposits of 

 Hingham are completely isolated by the drift formations and 

 the sea, — cut off, alike from the stratified rocks of Nantasket 

 on the east and those of Weymouth and the Blue Hills on the 

 west. Notwithstanding these difficulties, however, we have 

 two clues which are satisfactory, so far as they go, although 

 they are, unfortunately, not of such a nature as to lead to a 

 definite determination of the geological horizon. These are 



( 1 ) the relations to the older eruptives — the granitic rocks ; 



(2) the composition of the conglomerate. 



The Paradoxides beds of Braintree and the Blue Hills, 

 which Walcott now regards as of Middle Cambrian age, are 

 clearly intersected by, and therefore older than, the different 

 varieties of granite and felsite of that district. We have no 

 reason to doubt that these granitic rocks are the same for the 

 entire South Shore, from Scituate and Cohasset westward ; 

 and therefore it follows that the conglomerates of Nantasket 



