Crosby.] 10 [January 16, 



ate of the Boston basin lithological or stratigraphical peculiarites 

 which are so localized as to afford a proper basis for, or even sug- 

 gest, a chronological division. The most general distinctions are 

 those resulting from a gradual passage from the normal type of 

 conglomerate into finer varieties and sandstone and from these 

 into slate ; but here the evident gradation destroys the inference. 

 The slate, especially, is a very variable rock. The principal col- 

 ors are black, bluish black, gray, greenish, purplish and brownish 

 red. It may be very thin bedded and beautifully laminated, or 

 so massive as to appear unstratified. It may possess a well 

 marked cleavage or joint structure, or both these may be almost 

 entirely wanting. But none of these differences admit of correl- 

 ation with the distribution. Locally these rocks are subject to 

 great variations, but to the general view they are decidedly hom- 

 ogeneous. 



In my " Contributions " I have shown that the almost perfect 

 agreement in strike and dip, relations to the granite and litholog- 

 ical and mineralogical characters, of the slate in and near the 

 Paradoxides quarry, in Braintree, on the north side of Hayward's 

 Creek, in Quincy, on the east side of Weymouth Fore River, in 

 Weymouth, and along the Monatoquot River, places their essen- 

 tial identity beyond reasonable doubt. Between Quincy Point 

 and the Old Colony Railroad there are no outcrops ; but west of 

 the railroad the slate is exposed immediately north of the gran- 

 ite, and agrees in all important respects with that in the vicinity 

 of the Paradoxides quarry and Weymouth Fore River. This 

 comparative study has been extended over the entire basin, with 

 substantially the same results; the facts everywhere favoring the 

 view that the slates are all of the same age ; though, of course, 

 the proof becomes, on the whole, less conclusive the further we 

 recede from the fossiliferous beds. Occasionally, however, we 

 find unique evidence connecting the slates of widely separated 

 localities. Thus the slate forming East Point, Nahant, is, in its 

 relations to the eruptive rocks which border it and in the altera- 

 tion which it has suffered, identical with the slate at Mill Cove, 

 in Weymonth, and like that it holds thin beds of limestone. The 

 character of the limestone and its mode of occurrence, are the 

 same at the two localities, although thirteen miles apart ; and the 

 evidence of synchronism is much strengthened by the fact that 



