60 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



surface of erosion which underlies the Carboniferous in 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 determinative value have been found, although 

 Burr and Burke did find a fossil tree trunk in the Roxbury conglomerate proper. 1 



"HISTORY OF THE TILLITE. 



"A study of the sediments of the Boston Basin gives some idea of the physiog- 

 raphy 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 deter- 

 mined. 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 preserved 

 as a tillite, it must ordinarily be on a surface which is subsiding at or soon after 

 the time of the retreat of the ice-sheet. * * * Whether the slate above the 

 tillite is of marine or fresh-water origin it is not possible at present to say. No 

 clearly marine fossils have been found in it, and so far as this negative evidence 

 goes it is more probably that this slate is of lacustrine origin. The absence of 

 fossils, however, does not settle the question. Marine life in the Permian seas 

 was scarce or wanting altogether in many places, and furthermore, fossils are 

 not found in the marine clays of Pleistocene age outcrops. If volcanoes were 

 situated then as now near the continental margins, the sea might not have 

 been many miles away, for volcanic action was associated with the deposition 

 of these beds, as shown by melaphyre flows in several places in the basin. Ac- 

 cording to Bailey Willis (Jour. Geol., vol. 17, 1909, pp. 403-405), land extended 

 at least 100 miles in a southeasterly direction from Boston and probably much 

 farther than this. That there was high land to the southeast appears probable 

 also from a study of the tillite. The evidence so far points to a southeasterly 

 origin for the ice which formed the tillite. * * * 



"The Roxbury conglomerate proper at Atlantic exposes a thickness of about 

 520 feet. The lowest part shows rather small pebbles averaging about I inch in 

 diameter. Farther up the pebbles increase in size gradually, while in the transi- 

 tion beds below the tillite the pebbles are larger, averaging about 4 inches. It 

 would seem very probable that this gradual increase in the size of the pebbles 

 heralded the coming ice-sheet by wetter conditions or by a shorter distance from 

 the source, as the ice drew nearer. If the larger size of the pebbles was due to 

 more water and greater velocity, the pebbles should be as rounded as formerly, 

 but if the approach of the ice was the cause of the size, the pebbles should be 

 more angular as well as larger. The latter appears to be the case. 



"Above the Roxbury a sandstone bed was formed, indicating slower stream 

 action. A bed of conglomerate was then laid down, indicating swifter stream 

 action. Another sandstone bed was then deposited. At this point a new phe- 

 nomenon is met with. Above this last- mentioned sandstone comes a conglomer- 

 atic mass which differs from the Roxbury in having fragments and lenticular 

 layers of slate. * * *. From a study of this bed I infer that the ice had come 



1 Burr, H. T., and R. E. Burke, Proceedings Boston Soc. Nat. Hist., vol. 29, pp. 179-184, 1900. 



