ELEMENTS IN SAND-PLAIN FORMATION 401 
of salt or brackish water which rested against the ice on the 
north, and was practically cut off from the sea on the east by a 
till ridge, and on the south by the Nyatt Point sand-plain. Into 
this body poured.a glacial stream with a volume of some 
15,000 cubic feet per second, and bearing a load which I have 
assumed as a maximum to be 13 grams per liter, amounting to 
526,500 tons per day. From experiment it has been found that 
material like that of the Barrington clays settles very rapidly, 
indicating that practically the whole amount brought in by the 
glacial stream must have been deposited within the inclosed area 
indicated. 
The amount of the clay is some 95.3 milliontons. Dividing 
this by the daily discharge of sediment of the glacial stream, she 
time of the deposition of the clays is found to be 181 days, or almost 
exactly six months. 
General application. —Though in the sand-plains of different 
localities, the proportion of sand and clay varies greatly, the 
Barrington deposits taken as a whole probably represent very 
nearly the average conditions. I am satisfied, therefore, that the 
results obtained in the case of the Barrington plain, though 
strictly speaking they are applicable only to this plain, represent 
fairly closely the time required for the formation of the average 
sand-plain. If, for the reasons given ona previous page (p. 454), 
the ice retreat is considered as taking place in the early spring, 
it would follow that these figures represent a maximum, rather 
than a minimum, time limit. In the case of large plains, how- 
ever, with areas of several or many square miles it may be pos- 
sible to consider the period of deposition as extending over more 
than one season of melting, there being in the meantime either 
no retreat of the ice margin, or a retreat so slight that the inter- 
vening space was completely filled, and the sand-plains united 
into a single compound plain. 
Remarks. — One of the points most strongly emphasized by 
the results obtained, is the almost incredible amount of sediment 
discharged by the Barrington glacial stream during the few 
months of summer activity. The daily discharge of sediment by 
