44 SEASONAL DEPOSITION IN AQUEO-GLACIAL SEDIMENTS. 
the waters which supplied this part of the slate came from an area of melaphyre 
under the ice. I have assumed in this paper that the Squantum slate is the 
equivalent of the Cambridge slate. From present knowledge it appears to be, 
but if it should turn out that there are two different tillite formations instead 
of one this slate would belong to a different horizon. 
In 1911, while working on the Squantum tillite, I noted that the bands 
in the slate which overlies the tillite were much narrower near the highest 
observable part of the slate formation than near the bottom. The uppermost 
slate exposure appeared to have no bands at all. The reason for the gradual 
disappearance of the bands appeared to be in the gradual withdrawal of the 
glacier which supplied the material for the slate, to a great distance, thus reduc- 
ing the supply toa minimum. That this highest slate had extremely fine bands 
was not observed until several years later. Later on, it occurred to me that 
this banding might be due to seasonal deposition like the banding in the glacial 
clays of Sweden described by de Geer. When I examined the slate with this 
idea in mind I was pleased to find that the phenomena described by de Geer 
and others in the glacial clays were also present at Squantum in the slate. The 
resemblance between the banding in the clays of de Geer’s description, and the 
banding in the Squantum slate was so close that an account of these resemblances 
may serve to make clear some of the important points of this paper. 
The slate is well exposed at two places at Squantum; at the southern extrem- 
ity of the peninsula and at the northern extremity. The former will be called 
Squantum South and the latter is known as Squantum Head. There is also an 
exposure midway between the two, on the eastern side facing Quincy Bay, but 
as this slate all lies below high-tide level it is covered with a marine growth 
and the banding, although present, is difficult to study. 
I recalled that there was a gradual transition at the south exposure, from 
the thin conglomerate lying on the tillite, through layers of sandstone and slate, 
to almost pure slate; and from a reinvestigation, it was clear that this transi- 
tion from coarse to fine material upward was exactly like the transition which 
de Geer described. On the tillite lay some coarse waterlaid conglomerate with 
rounded pebbles, some of them eight inches in diameter. This conglomerate 
may be fifteen feet thick. The Pleistocene drift between it and the next exposure 
makes it difficult to determine whether it is one bed of conglomerate or several, 
separated by beds of finer material. About fifteen feet above the tillite, three 
feet more of conglomerate are visible, of a little finer texture, then comes a . 
bed of sandstone four feet six inches thick. From this point upward in the 
