428 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



■where shown its chemical characters. I have pointed ont, in the preceding 

 pages, that it distinctly overlies the Norian rocks ; and it no less clearly under- 

 lies the Mont Alban. It exhibits much greater disturbance than the Mont 

 Alban, but less than the Norian ; and near the common boundaries of this 

 series and the Mont Alban, the gneiss of the latter is cut by the eruptive gran- 

 ite and diorite of the former. And since this series so closely resembles, in its 

 internal cliaracters and its external relations^ the Huronian of other regions, 

 we arc bound to conclude that it is the Huronian." 



Of the argillite and conglomerate in the vicinity of Boston, Mr. 

 Crosby remarks (1. c, pp. 40-42) : 



*' Although the fossils characteristic of the Acadian groiip have been found 

 at only one locality in Massachusetts, viz., Hayward's quarry, in Braintree, 

 yet most observers agree that the greater portion of the slates in the vicinity 

 of Boston are probably of Primordial age ; and I have so represented, them on 



the map The conglomerate so well developed about Boston, and widely 



known as the Roxbury conglomerate, is lithologically identical with the car- 

 boniferous conglomerate of Bristol county and B,hode Island, and seems to be 

 similarly related to the Primordial rocks. It has been frequently referred to 

 the Carboniferous horizon ; and, in view of the facts just cited, and the absance 

 of any positive evidence to the contrary, this is certainly the most probable 

 view of its\age." 



Dr. Wadsworth published, in 1877, a description of ttie dikes in cer- 

 tain portions of the Boston basin, based on a study of their field 

 relations and microscopic characters. He showed that the siipposcd 

 hornblende was in general augite, and that the rocks v^gyq old basaltic 

 ones belonging to the melaphyr and diabase varieties. It was also 

 proved that dikes of two, if not three, distinct periods existed here, and 

 that the later faulted the earlier ones. In 1878 he also showed that 

 the Eockport granite-^ was of two varieties, a micaceous and a horn- 

 blendic one. These were found by careful observation on continnous 

 exposed surfaces to pass into one another, usually having an intermedi- 

 ate grade containing both hornblende and mica. (Proc. Best. Soc. Nat. 

 Hist., XIX., pp. 217-237, 309-316.) 



In a paper "On a Possible Origin of Petrosilicious Rocks" (Proc, 

 Best Soc. Nat. Hist, 1879, XX., pp. 1G0-1G9), Mr. Crosby advocated 

 the view that the felsites were of a deep-sea origin corresponding to the 

 " red clay " of the Challenger Expedition. In this paper he remarks : 



" These rocks, which are widely distributed over the globe and compose 

 formations of great extent, are undoubtedly of marine origin. We can scarcely 



J 



regard them as shore deposits, and therefore it seems natural and legitimate to 

 conclude that they were formed^ in the deep sea ; and I w^ould submit that 



