18 BULLETIN OF THE 



Attleborough, also exhibit northwest and southeast strikes. This matter 

 I have considered in some detail in my Report on the Geology of Mar- 

 tha's Vineyard, now in press in the United States Geological Survey. 



The age of the rocks which I have terraed Pre-Cambrian must for the 

 present remain doubtful. The absence in the section of any beds like 

 those containing the Paradoxides of Braintree raises the presumption 

 that they do not belong in the Braintree Cambrian series. The massive 

 limestones which occupy a portion of the section are also to a certain 

 extent evidence to the same effect. As a whole this section reminds me 

 more of that which occurs at Eockport, Maine, than any other deposits 

 known to me on the coast ; still I think there is nothing which can be 

 evidence to prove the likeness in age of these beds. 



General Character of the Cambrian Rocks. 



As the object of this paper is to set forth the phenomena of the 

 Cambrian series with no other attention to other deposits than is neces- 

 sary to make them comprehensible, I shall now proceed to give in some 

 detail an account of the deposits which appear to belong in this portion 

 of the section. S^: far fossils have been found in rocks of this section 

 which probably do not in the aggi-egate include more than one hundred 

 feet or so of the total section of the Cambrian series. However, as these 

 deposits are of the same aspect as all the red slates and conglomerates 

 of the area, it appears at present reasonable to include all rocks of this 

 description with the above-mentioned series. The total thickness of the 

 section which I have termed Cambrian is not accurately determinable. 

 It probably amounts to not far from two thousand feet. In the main it 

 consists of thin-bedded shaly layers which occasionally pass into moder- 

 ately thick fine-grained greenish and i-eddish slates. Intermingled with 

 these in several levels we have a number of layers of conglomerate, per- 

 haps as many as half a dozen distinct beds, varying in thickness from 

 two hundred to three hundred feet. In all cases these conglomerates 

 are frequently interrupted by thin layers of shale or sandstone. The 

 pebbles are mostly of small size ; none have been observed exceeding a 

 foot in diameter, and few above six inches in thickness. The pebbles 

 are in most cases rather angular. At certain points they have a very 

 high degree of angularity, so that they assume the form of a breccia. 

 The rocks from which the pebbles were taken are mainly identifi- 

 able in the western portion of the field before described. Xo distinct 

 traces of cross-bedding have been observed in the deposit. Indeed, in 



