12 The American Geologist. July, 1894 
younger, but more perfect, of the two type-specimens) forms 
a rib-like thickening. 
Mi asurenu nts. — Hight 21 ; breadth of bocly-whorl 11.5 mm : 
divergence of slopes 52 degrees, — these being the measure- 
ments of a young shell. An imperfect specimen, which per- 
haps represents nearly the adult size, indicates a hight of 
nearly 50 mm. 
Occurrence. — In No. ."! of the Belvidere section, about ;i mile and a 
half south of I lie railway station. 
There are 14 of t he coarse vertical folds on the body-whorl and 13 on 
the firsl spire-whorl in the smaller of the type-specimens: 
I am unable to refer this shell elsewhere than to the upper Jurassic 
genus, Petersia. 
CAUSES AND CONDITIONS OF GLACIATION. 
By War rex Upha.w, Somerville, Mass. 
At the meetings of the British Association in each of the 
past two years the causes of the Glacial period have been dis- 
cussed from new points of view. One of these papers, pre- 
sented in 1892 by Percy F. Kendall and J. W. Gray, is fully 
published by Mr. Kendall in the Transactions of the Leeds Geo- 
logical Association for Feb. 16. 1893 (pages 53-70). This 
paper holds that the Glacial period came on with extreme 
slowness: that it was of long duration (an estimate of at 
least about L 1,000 years being given for the time of growth 
of the European ice-sheet, and two-thirds as long for the 
North American) ; that the Glacial period ended very abrupt- 
ly ; that the level of the British Isles was nearly the same at 
the beginning of the Glacial period as now; that the end of 
this period was very recent in a geological sense (the esti- 
mates of 10.00(1 years, or less, for the Postglacial epoch, as 
drawn from the rates of recession of the falls of St. Anthony 
and of Niagara, being accepted); and that there has been 
only one epoch of glaciation. This review of the Ice age for- 
bids an explanation of its causes by the astronomic theory of 
( 'roll, Geikie. and Ball. Against the American theory that 
great uplifts of the drift-bearing lands brought on their cool 
and snowy climate and glacial envelopment, and that final 
subsidence under the weight of the ice-sheets caused them to 
