152 bulletin: museum of comparative zoology. 



termined. While most of these are considerably altered, either to 

 sericite or to calcite, as the ease may be, a few are remarkably clear 

 and fresh. 



" Now, as regards the origin of this tillite, there are two agents which 

 have been ascribed to its formation, — vulcanism and ice. If this 

 were a product of extrusive action, it would be called a tuff or an 

 agglomerate, and in either case it should reveal signs of a former glassy 

 nature of all or part of its components. To my mind, there is no sug- 

 gestion of such an original structure. The rock does not at all re- 

 semble the tuffs and agglom.erates found elsewhere in the Boston 

 basin. On the other hand, if this were a tj'pical till in a consolidated 

 state, its finer parts should be composed largely of rock-flour; kaolin 

 should not be an abundant constituent. Unfortunately the finer 

 portion of the feldspar elements has gone to form sericite and calcite, 

 and the original source of these secondary minerals is therefore not 

 determinable; but the considerable amount of larger feldspar grains, 

 many of them very little altered, suggests that kaolin was not abundant 

 originally. 



" In conclusion, then, I may say that the megascopic and microscopic 

 study of this rock lead me to believe that ice was the most important 

 factor in its deposition; but that water, too — standing or gently 

 moving — was concerned in its origin. I could find no evidence of 

 contemporaneous erosion throughout this section. Both upward 

 and downward, stratigraphically, in the section, the tillite beds grade 

 into the water-laid strata. Apparently the ice Avas either floating or 

 had its weight much reduced by partial flotation." 



In the above letter Dr. Lahee suggests possible flotation for the 

 ice. I have not found any evidence that such was the case. In view 

 of the large fragments of slate in the tillite, and the disrupted beds 

 found at Squantum Southeast, not to mention the immense block of 

 sandstone, fifteen feet in diameter, which is evidently part of a dis- 

 rupted bed, I cannot agree with Dr. Lahee on the flotation idea. 

 Again the first beds encountered at the top of the tillite are conglomer- 

 ate of coarse texture, sandstone layers, then sandstone and slate, and 

 last slate without sandstone. These beds would indicate that the 

 water in front of the ice was at first shallow, and the slate would 

 indicate that subsidence was in progress as the ice retreated. 

 This transition may be seen to best advantage at the most southerly 

 part of the Squantum Southeast exposure where the slate appears on 

 the shore. 



Locality 12. Squantum Southeast. At the end of the road running 



