740 Jal Nile Sey IN UNI ESS TBI 
3600 years, or, in the aggregate, 14,400 years. The total, 
therefore would be, in round numbers, about 22,000 years for 
the mere transportation of a single erratic in each invasion, and 
that at an improbable rate of speed and without any allowance 
whatever for the time occupied in the formation, culmination, or 
retreat of the glacier, or for interglacial periods. It is suffi- 
ciently evident from these figures that the ice-sheet theory of 
the till formation is utterly incompatible with such estimates as 
those of Prestwich and Wright, which give the whole glacial 
period a duration of only 20,000 to 40,000 years. 
It has been matter of surprise to me that so little weight has 
been given by authorities to the arguments from the calculation 
of the transportation of erratics in the estimation of the dura- 
tion of the glacial period. Most of them absolutely ignore, or 
at least fail to utilize it, and those who do allude to it at all, 
like Helland,’ give it only the briefest and most casual men- 
tion. It appears to me to be the one method by which we can 
obtain, not the actual, but the utmost possible minimum of 
duration of such an ice-sheet as the generally accepted glacial 
theory demands. 
Professor W. J. Crosby* has offered the suggestion that, as 
the great mass of the rock débris of the till is local and has never 
traveled far from its place of origin, the northern erratics were 
transported largely by water in the glacial lakes that formed 
along the borders of the ice-sheet. Inasmuch as these are found 
throughout the till at all levels, his suggestion amounts practi- 
cally to an admission that the whole mass in which they are dis- 
seminated was thus deposited, which is altogether inconsistent 
with the general tenor of his argument, and is almost, if not 
quite, equivalent to giving up the land-ice theory of the deposi- 
tion of the till: 
It may be worth while here to notice one or two estimates 
or statements in regard to the duration of certain stages of the 
glacial period by prominent glacialists. The recent estimate of 
t Zeitschr. der deutschen Geologischen Gesellsch. XXXI, p. 76, 1879. 
2Am. Geologist, XVII, 1896, p. 234. 
