SAYLES: THE SQUANTUM TILLITE. 165 



preserved as tillite, it must ordinarily be on a surface which is sub- 

 siding at or soon after the time of the retreat of the ice-sheet. Any 

 till deposit above sea-level on a stationary or rising surface would 

 almost invariably be eroded long before later subsidence could remove 

 it beyond the wear and tear of the elements. 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 probable 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 

 everywhere in the marine clays of Massachusetts and ]Maine and other 

 places where marine clay 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. According to Bailey Willis (1909, p. 

 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. A discussion of this question of direction 

 comes naturally in the history of the appearance of the tillite as shown 

 best in the Atlantic exposure, and in a study of some features of the 

 tillite found at the southeastern Squantum exposure. 



The Roxbury conglomerate proper at x\tlantic exposes a thickness 

 of about 520 feet. The lowest part shows rather small pebbles averag- 

 ing about one inch in diameter. Farther up the pebbles increase in 

 size gradually, while in the transition-beds below the tillite the pebbles 

 are larger, averaging about four inches. It would seem very probable 

 that this gradual increase in the size of the pebbles heralded the com- 

 ing 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 phenomenon is met with. Above this last men- 



