Perry.] 124 [February 28, 
which is exceptional and still deserving of attention. Many mate- 
rials were laid down, even in a more regular form than those just 
mentioned, and have been, either with the preceding or by them- 
selves alone, very generally regarded as a marine deposit.. A few 
sentences will perhaps make clear the true view of their origin. in 
connection with the wasting of the main ice-sheet, there was the 
occasional leaving behind of great heaps of detrital matter, which 
had been accumulated before the advancing mass of ice. ‘There 
must have been from year to year, or from one series of years to 
another, during the Later Glacial times, alternate advances and re- 
cessions of the ice-fiéld. Connected with the forward movements, a 
vast amount of matter would be in many cases pushed forward and 
heaped up in front of the ice-mass. On its retreat, we should expect 
these heaps to be left in the form of terminal moraines. In the ease 
of narrow defiles and gorges, the accumulations of matter must, in 
some cases, have been enormous; and these would remain after the 
wasting of the ice, remain even after the ice had made a long stage of 
retreat. Under such circumstances, as we see at a glance, these 
piles of heterogeneous material must have occasionally served as 
barriers, as natural dams, checking up the valleys, and forming basins 
for lakes, broad ponds and wide river-areas. Now the waters flow- 
ing, as usual, from the wasting ice in great abundance, were likely to 
fill the new-formed reservoirs. These waters, issuing from the end 
of the wasting mass, and working over the material in their way, 
were necessarily turbid. Passing on with their burden of comminu- 
ted matter, they would deposit, in some places clay, in others sand, 
and these in beds more or less extensive, according to the area of 
the basins. Of course, the waters seeking the lowest depressions, 
must all along in their southward course lay down deposits, but most 
especially where they came to a stand-still. Hence some of the beds 
would naturally have a southern slope; while usually the most dis- 
tant deposits, if made in a basin, would be horizontal and consist 
mainly of clay, this being the finest material, and accordingly most 
readily transported. It is clear that the waters might make such 
depositions to some extent, where only a few obstacles impeded their 
progress, but particularly when they were obstructed by barriers, 
and thus made to cover broad areas. So it is equally evident that 
the barriers wearing away, or being from time to time broken down, 
the waters would have varying levels, and the deposits reach unequal 
heights at different parts of the progress of the work ; at times, also, 
