ELEMENTS IN SAND-PLAIN FORMATION 455 
over a large area as an inconspicuous sheet. (6) In the 
summer the ice margin would become stationary and the depo- 
sition of the detritus, which was derived almost entirely from 
the ablation of the débris-laden ice, would be concentrated at 
definite points. ; 
Although from the above considerations it seems reasonable 
to refer the shorter ice retreats between sand-plain stages to the 
spring months, it is probable that the longer retreats represent 
longer periods, perhaps in some cases years in length. In this 
case the absence of inter-sand-plain deposits may be more 
seeming than real, because of the sheet form of such deposits. 
TIME ELEMENT 
If the months of December, January, and February are 
eliminated, as there can be no doubt they should be, from the 
time of sand-plain growth, the retreat of the ice and the deposi- 
tion of the plain must have taken place inthe remaining nine 
months. The distance between sand-plain stages varied from a 
fraction of one up to several miles, and three months certainly 
seem none too long atime for sucha retreat. If this be so, 
the sand-plains, or at least those of modern size, must have been 
formed in the six months still remaining. 
A study of the conditions now existing in the larger glaciers 
of Alaska, showed at once that sucha rate of deposition was by 
no means improbable. A calculation based on such conditions 
could not fail to be of interest, and would give valuable indica- 
tion as to the probability of the general estimates. The results 
showed an unexpected and surprisingly close agreement with 
the estimates. 
Basis of calculations —Evidence as to the time of formation 
of sand-plains is afforded; (1) by the bulk of the sand-plain 
itself, and (2) by the bulk of the accompanying clays, which 
from their mode of formation, are known to be simultaneous in 
development and coextensive as to time. The time estimate is 
obtained by dividing the bulk by the daily discharge of sedi- 
ment of the glacial stream. To find this discharge of sediment 
