GEOLOGY: R. IV. SAYLES 
169 
in well banded slate. The second tillite bed is about 150 feet from the 
main tillite. Each of these beds is about 5 feet thick. From the evi- 
dence presented by these tilKte beds and several other beds of con- 
glomerate in the slate, it is inferred that the final retreat of the glacier 
was slow and hesitating, marked by several advances after many years 
of retreat. It is known that the ice of the Wisconsin epoch of the 
Pleistocene period also began its retreat very slowly. 
The cause of the disappearance of the sandstone layers and the 
gradual thinning of the bands can be explained by the retreat of the 
glacier and the consequent slackening of the currents, which would be 
strong enough to carry sand only in the neighborhood of the ice. The 
lower pebbly members of the transition beds show irregularities of de- 
position, especially in lenticular forms, due to the inconstant condi- 
tions of streams coming from a glacier. A regularity of alternation in 
deposition, however, becomes evident after the first 50 feet or so of the 
transition beds have been passed, where the layers indicate deeper and 
quieter water, and thus more uniformity in deposition. The thin indi- 
vidual layers now show through hundreds of feet such regularity in thick- 
ness and interval that a regularly recurring cause must be sought. 
At the International Congress of Geologists at Stockholm in 1910, 
Gerard De Geer read a paper on the banding of the glacial clays in Swe- 
den. He thinks, that the coarse band or layer gives a record of summer 
melting and deposition, and that the find band or layer gives the record 
of winter deposition of fine material, when the streams were slow and 
the fine material could easily settle on the bottom. With the co-opera- 
tion of his students he was able to count the layers in the late glacial 
clays and then in the post-glacial deposits of the extinct Lake of Ragunda, 
which was drained in 1796, and gives 12,000 years as the time elapsed 
since the ice retreated from southern Scania to its present northerly 
position. The idea of measuring geological time in this manner came 
to De Geer in 1878. In this country B. K. Emerson advanced the same 
idea regarding the layers in the glacial clays of the Connecticut Valley 
in 1898.^ Since that time several American geologists have pubHshed 
the theory. Among them may be mentioned A. P. Coleman, Frank 
B. Taylor, and Charles P. Berkey. As far as I can learn, however, 
De Geer is the first to conceive this geological time recorder, and also 
the first to convince the larger part of the geological world that these 
double bands in the glacial clays really mean years. No other expla- 
nation so far advanced, accounts for all the facts of the case. The 
facts observed in the slates at Squantum resemble so closely those de- 
scribed by De Geer and Emerson and the others, that it would seem as 
