164 bulletin: museum of comparative zoology. 



already described, but with this are mingled many angular fragments 

 of amygdaloidal la\a and a few of granite and gneiss. Much of the 

 moraine has been removed by streams flowing from the glacier, but 

 remnants 2000 feet or more in thickness extend nearly across to the 

 highland north of the valley." (Russell, 1901, p. 106). 



Although striated rock fragments might be found in a mud-flow, 

 I have yet to find a reference in the literature to such, from an actual 

 mud-flow. Even if striated stones were found, it is not likely that all 

 the other evidence of glaciation would be found. Those who would 

 have the tillite under discussion a mud-flow, have also the onvs pro- 

 handi on their side. 



The age of the roxbury series. 



The exact age of the tillite is still uncertain. The lithological 

 characters of the Roxbury series resemble closely those of the Carbon- 

 iferous and Permian of the Narragansett and Norfolk Basins. The 

 Roxbury series, which consist of the Roxbury conglomerate, the Squan- 

 tum tillite, and the Cambridge slate, is newer than the Cambrian as 

 proved by pebbles in it of the granite which cuts the Cambrian. The 

 Roxbury series lie, without much doubt on the same granitic surface 

 of erosion which underlies the Carboniferous of the Narragansett 

 and Norfolk Basins. 



All that can be said at present is, that the tillite is of Permo- 

 Carboniferous age. The fact that the Permian glaciation was so 

 widespread, and that new evidence of it is coming in so rapidly, makes 

 it very probable that the tillite is of Permian age. No fossils of de- 

 terminative value have been found, although Burr and Burke did 

 find a fossil tree-trunk in the Roxbury conglomerate proper. (Burr, 

 H. T., and Burke, R. E., 1900, p. 179-184). 



History of the tillite. 



A study of the sediments of the Boston Basin gives some idea of the 

 physiography of the region, during late Carboniferous or Permian 

 times. The area in which the sediments were deposited extended 

 far and wide beyond the present limits of the deposits. That the 

 area of deposition was low relatively to the surrounding country is 

 certain, but that it was at sea-level is not so easily determined. 

 Towards the close of deposition the land must have been subsiding as 

 shown by the thick bed of slate over the tillite. In order for till to be 



