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62 SEASONAL DEPOSITION IN AQUEO-GLACIAL SEDIMENTS. 
slate belongs to just the same age as the Squantum slate. All these other 
outcrops of the Cambridge slate need very much more study in order to identify 
horizons, for it is very evident that climatic conditions suitable for seasonal 
deposition in one part of a slate formation might not exist in another part. 
Furthermore, slate without apparent banding may be well banded, as the 
experience with the slate under the tillite at Squantum has shown. (See p. 41). 
It is needless to go over the points of similarity between the structure of 
the Grantsburg clays as described by Berkey and my description of the struc- 
ture of the Squantum slate. If there is any difference in the structure of the 
two it is not possible, after reading Berkey’s description, to discover it. Pro- 
fessor Berkey has not seen the slate in situ but he has some of the photomicro- 
graphs, and believes that I am justified in interpreting the features shown in 
the slate as similar to those of the banded clays. Mr. Frank B. Taylor spent 
two days with me examining the slate formation and the microscopic sections 
of the banding. He has been kind enough to allow me to quote his opinion 
which is as follows: 
“T am not only willing to be mentioned as a visitor to your Squantum locality, but 
would be glad to be listed among those who believe in the general correctness of your inter- 
pretation of the banded slates as due to seasonal variations.” 
Professor Barrell has spoken with enthusiasm of the banded slate at Squan- 
tum; he considers (1917, p. 828, 902) the structure of the slate banding and 
clay banding the same. Professor Hobbs, who was with de Geer on the 1910 
field trip to the Swedish localities, has stated (1916, p. 112) his opinion regard- 
ing the similarity between the Squantum slate and the banded clays. 
Evidence of seasons in the Permo-Carboniferous have come from New 
South Wales, Australia, in the discovery of annual rings of growth in trees of 
Permian age. (Shirley, 1898, p. 14, 15; Arber, 1905, p. 36, 37). From Brazil 
similar evidence of seasons was obtained in fossil trees of the same age. (I. C. 
White, 1908). Physicists have shown, that so far as can be judged from any 
known evidence, polar wandering, at least to any appreciable extent, has not 
taken place during the known geological history of the earth. (Barrell, 1914, 
p. 333-340). 
Blackwelder has argued that there are ancient glacial clays without associ- 
ated tillites. (See ante, p. 58). Itis probable that many more await discovery. 
As has been noted in a former paper (Sayles, 1916), the chances for the preserva- 
tion of clays are much greater than for tills, for the reason that clay is deposited 
