152 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
termined. While most of these are considerably altered, either to 
sericite or to calcite, as the case 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 agglomerates found elsewhere in the Boston 
basin. On the other hand, if this were a typical 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. 
“Tn 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 was 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 
