92 Correspondence. 
They occur in the following natural order :— 
3rd. Gravels and sands, including Eskers and 2 Post-drift Gravels 
Kaims d ; 4 ‘ i JS : 
2nd. Clay and blocks, usually made up chiefly 
of the débris of the underlying rocks, 
but sometimes consisting almost en- | Boulder-clay, or 
tirely of limestone fragments: in this Glacial Drift. 
latter case, the material is locally called 
‘Corn Gravel’ 
Ist. Gravel, sand, and clay; the last contain- 
ing fragments of plants, &c. : : ‘ Pre glacial te 
In Ireland I do not remember to have seen a section in which 
these three kinds of drift are represented, but in many places I have 
found Nos. 2 & 8, and in a few Nos.1 &2. No. 2 is undoubtedly 
Glacial Drift, as it was deposited from the large sheet of ice that 
once covered the country ; while No. 1 must have been previously 
deposited by water, or accumulated on the land; and No.3 was 
formed from the part of No. 2 that was washed by and deposited 
in water. In both Nos. 1 & 3 I would expect to find Arctic Shells 
and erratic blocks, as they were formed in a similar manner to what 
is now going on in the Arctic and Antarctic Circles. ‘There, in the 
large fields of ice, the materials for the Boulder-clay are accumu- 
lating ; while in the seas around gravels and sands, with Arctic 
Shells, are being deposited; and the droppings from the passing 
icebergs supply the erratic blocks. If the land is rising, the field of 
ice, and consequently the Boulder-clay, will extend out over these 
sand-deposits; but if the land gradually sinks, part of the Boulder- 
clay will be washed into gravels and sands; and, as they still con- 
tinue to be in an Arctic sea, there will be similar shells mixed with 
them, and passing bergs will supply the erratics. If the ice-field 
does not reach the coast-line, plants, &c. will grow on the inter- 
vening land, which will be destroyed and covered up by the Boulder- 
clay, if there be a continuation of severe seasons, and the ice-field 
extends beyond its usual limits. Recently I have found a section in 
the Baleyneenadouish River-valley, near Gort, Co. Galway, in which 
there is Preglacial Drift, under about twenty-five feet of Boulder- 
clay. The Preglacial Drift consists of clay and fine sand, and con- 
tains sticks, fir-cones, &c. This section I hope fully to describe in 
a forthcoming memoir of the Geological Survey. 
In the Preglacial Drift Ihave never found striated blocks ; but I 
do not say that they do not occur, as they might have been dropped 
into it from passing bergs. The surface of the rocks under it I 
never found polished or striated; but, when the true Bowlder-clay 
lies without any intervening rubble, the rock-surfaces are always 
polished and striated. The Post-drift-gravels may lie on a ‘ dressed’ 
rock, but the polishing and striz are always obliterated; rounded 
blocks may also occur in it; but the polishing and scratches are 
always more or less obscure, and never have the fresh look of the 
