EI.ISHA MITCHEI^I^ SCIENTIFIC SOCIETY. 39 



deposition. This difficulty is increased by the removal 

 of the o-reater part of these rocks by erosion, leaving- 

 widely separated patches which can scarcely be placed 

 in any reasonable relation with each other. Usually, 

 however, the series lie upon the eroded surfaces of pre- 

 Cambrian rocks, Laurentian and iVlgonkian, and often 

 show basal cong-lomorates formed from those rocks. In 

 such cases the materials seem to have been broug-ht 

 from the west or northwest, and from no great distance. 

 These areas lying about the Gulf of St. Lawrence 

 seem to show by the gradual chang-e in the character 

 of the sediment from basal conglomerate to limestones 

 formed at a moderate depth, that the sea transgressed 

 the Algonkian land westward, allowing^ the accumula- 

 tion thereon of the Cambrian deposits, first in shallow 

 bays and afterward in gradually deepening- water. 



Reg-arding the Cambrian slates and quartzites of 

 eastern Massachusetts, Prof. W. O. Crosby says; "In 

 general the quartzite is more and the slate less abun- 

 dant northwestward, indicating that the ancient shore 

 line along- which these slates were deposited lay in that 

 direction, and originally the Primordial strata were 

 probably spread continuously over all the region to the 

 southwest of that line. " Also, " It is very clear that 

 the quartzite, north and west of the Boston basin, is 

 the source of the quartzite pebbles which play such a 

 prominent part in the composition of the conglomerate, 

 especially in the central and northwestern sections of 

 the basin." 



Of the Cambrian section of Bristol county, Massa- 

 chusetts, Prof. N. S. Shaler says: "The frequent 

 return of conglomerate layers and the coarseness of 

 the pebbles show that during- most of the time when 

 the beds were accumulating- the region was near shore; 



