168 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



permanently from the Basin, and the land had subsided, and continued 

 to subside until several hundreds of feet of clay had been deposited. 



The direction of movement of the glacier which produced the tillite 

 is most important. There are a number of considerations which in- 

 dicate a direction from the southeast to the northwest. Though not 

 certainly due to ice thrust, the plication of the intercalated bed men- 

 tioned above, points to such a direction of movement. Again in 

 the description of this locality (page 155) it should be noticed that the 

 beds intercalated in the tillite strike at an angle of from eight to ten 

 degrees more east than the main bofly of the slate higher up. This 

 must mean either a diastrophic change in the attitude of the beds, or 

 that the intercalated beds sloped downwards towards the level of the 

 water in which the slate was deposited. There is no evidence of an 

 eroded zone between the transition-beds and the slate, so it does not 

 appear that there is any unconformity. The beds in question slope 

 from the east towards the west. According to Prof. James Geikie 

 (1895, p. 24), beds intercalated in till are diagonal and not as a rule 

 horizontal, and slope towards the ice-front. It would appear that the 

 beds in the tillite at Squantum Southeast dipped westward, and if this 

 was the case, and the difference in strike is not due to diastrophic move- 

 ment, there would seem to be good reason for believing that the ice 

 came from an easterly direction. Then again, a consideration of the 

 slate fragments might also indicate an east-west direction of ice move- 

 ment. In the tillite at Hyde Park, Milton Upper Mills, Roslindale, 

 I have not observed slate fragments. At Squantum, and Atlantic 

 the rock fragments in the tillite show a majority of pink granite, with 

 melaphyre and quartzite coming next in abundance. If the ice had 

 come from the north, the granite fragments could be explained, but 

 not the melaphyre. If it had come from the west, the melaphyre 

 fragments could be explained, but no pink granite of the variety found 

 in the tillite is known in that direction. If the ice came from the south 

 the pink granite could be accounted for, l)ut not the melaphyre. If 

 the ice came from the southeast, however, both the pink granite and 

 the melaphyre are explained, for at Nantasket, Cohasset, and Hing- 

 ham these rocks are found in situ. The fact that the largest 

 boulders found in the tillite are of pink granite and melaphyre, and 

 that these are found together, suggests a place of origin for both near 

 the same locality. I have not forgotten that Pleistocene drift may 

 hide some outcrops, and that the above suggestion cannot be proved, 

 but so far as known outcrops go, it is a legitimate speculation, and when 

 joined to the other evidence of the direction of ice movement appears 



