Origin of Eskei's. — Crosby. 39 
gesting-, as long ago as 1872, that the till or bowlder clay was 
mainly or wholly englacial during the maximum stage of gla- 
ciation, that it became largely superglacial during the final ab- 
lation of the ice, and that eskers were formed in superglacial 
channels. I am disposed to regard basal melting also as an 
important factor in releasing and depositing the ground mo- 
raine, and conceive that only thus have drumlins and the true 
"hard pan" been accumulated. But with this exception, the 
present argument may be regarded as a defense and elaboration 
of professor Winchell's view^s, which, although based pri- 
marily upon earlier work done in Ohio, were first published in 
the Annual Report of the Geological and Natural History Sur- 
vey of Minnesota for 1872 (p. 62) and in the Proceedings of 
the American Association for the Advancement of Science for 
the same year, (pp. 158 and 159), and again in the Popular Sci- 
ence Monthly for March, 1873, and more recently (1884) in 
Vol. I of the Final Report of the Alinnesota Survey (pp. 665- 
669), where the kame (esker) of Rice county is described and 
discussed. To Upham, however, we are still indebted for the 
vital principle that the ice floor of a superglacial stream may be 
eroded far below the base level : and it is not too much to say 
that without this contribution the superglacial argument would 
be still very inconclusive. 
ON THE DECEPTIVE FOSSI LIZATION OF CERTAIN 
PELECYPOD SPECIES AND ON THE 
GENUS EURYMYA. 
F. W. Sardeson, Minneapolis 
The dififerences in preservation as fossils of molluscan shells 
of calcite and aragonite are familiar to geologists. The less 
destructible shells may be the only ones to resist maceration 
long enough to bury under slowly building sediments and are 
then alone fossilized ; or where all shells have fossilized some 
are preserved, while the more soluble ones occur only as cast 
and mold ; or further, changes within the rock stratum may have 
altered or reduced the ones and have obliterated the others. 
In the case of Pelecypoda of the Paleozoic age, tho all must 
have had shells and all were capable of fossilization, yet not 
being equally so, some species mav be found and other species 
may be absent in a given stratum. Also in the many pele- 
cypods which had double layered shells, the one part either 
the inner or the outer, may remain after the other i)art of the 
