UPPER CAMBRIAN PEBBLES. Ill 



the drainage from the ice which were concerned in the distribution of the 

 pebbles seem to demand, in our present knowledge, au origin from some 

 point in Massachusetts Bay, as will be seen from an examination of the 

 accompanying outline map (fig. 5) of the lines of glacial flow in eastern 

 Massachusetts during the time of formation of the Cape Cod moraine. It 

 is true that in the closing stages of ice retreat there are indications about 

 Boston of a more easterly set in the ice, as if it were controlled by the local 

 slope to the sea. If this movement extended as far south as the Duxbury 

 shore, the derivation of the pebbles from the northernmost areas of con- 

 glomerate may be accounted for; but these fossiliferous pebbles have not 

 been found as yet in the Norfolk and Boston areas, either in the con- 

 glomerates or in the glacial drift, though quartzite pebbles abound. Glacial 

 stria? on a granitic ledge in Marshfield gave a reading of several degrees 

 west of north, but this locality is in the region of the Plymouth inter- 

 lobate moraine, and does not indicate that the ice moved beyond this line 

 of accumulation in this direction to the eastern shore of Cape Cod Bay. 



The Obolus pebbles are very abundant in the beaches of the south 

 coast, as far east as Nantucket and as far west as Block Island, being 

 nn ist common in the intermediate area on Marthas Vineyard. The pebbles 

 have been dispersed southward by the glaciation of the Carboniferous 

 ledges of the mainland. 



The earlier reference of these fossiliferous quartzites to the epoch of 

 the Potsdam sandstone has recently been confirmed by Walcott, 1 who states 

 that the brachiopods have their closest affinity in the upper Cambrian 

 fauna of the Newfoundland area. As vet the beds have not been found 

 in situ, an 1 little else is positively known regarding the upper Cambrian 

 formation of this portion of New England than that which may be inferred 

 from a collection of these pebbles. The information thus obtained may be 

 briefly stated as follows. 



Neglecting the question of superposition and the alternation of similar 

 beds with like faunas, data for which matters are of course wanting in the 

 pebbles, the upper Cambrian of the area whence the pebbles came appears 

 to have been composed of at least these three biological divisions: 



1. An Obolus zone of light-colored quartzites. — The pebbles of this zone exhibit 

 bands, 3 or 4 iuches thick, of these gregarious shells, usually preserved as black gra- 



1 Letter to Professor Shaler. 



