Geology and Mineralogy. 265 
tion of the poe in the eastern dunes, I incline to think it lasted 
500 or 1000 
(3.) The ware rose again, submerging for a short time the upper 
beach, but soon fell to the line of the middle one, where it remained 
about 1 ,600 or 2000 years. This period appears to be cotemporary 
with the Loess. 
4.) The water, which had already slowly fallen some feet, now 
retired more rapidly to near its present pions nae ch it has main- 
tained with only moderate fluctuation 
5. e total time of all these “deposit mae to be some- 
where pekwnes 5,300 and 7,500 yea 
The discussion is an interesting te important one. Some 
certainties in the calculations occur to us; but without a poe 
tion any but the following. The or writes as if he supposed 
that sand in the course —. transportation always remained sand. 
He observes that “the sand movement in the lake 3 is vended to 
ain under 
that while the ime material made ae this attrition is floated off 
the pany waters, the coarser is left behind in such cases near or on 
the shor 
8 f 
clther Hydrobia, Planorbis, nba oes Valvata, Vertigo, all 
species Valvata anomala Moore, and Planorbis Mendipensis 
Am. Jour. Sct.—SEconp Srries, Vou. L, No. 149.—Sxpr., 1870, 
17 
