84 I lie /hllCricail (iCologist. Fobruary, 1898 
the true explanation. It regards the shell fragments as de- 
rived by glacial transportation from the area of the Irish sea, 
by an ice-sheet flowing southward from southwestern Scot- 
land, northwestern England, and northeastern Ireland, by 
which the early Pleistocene beds of that sea basin were plowed 
up and mingled with boulders from the more distant north- 
ern tracts of thick ice accumulation. Many of the boulders, 
with much finer drift, were derived from mountains high 
above the old sea bed; but I think that drift from all these 
sources, both high and low, was intermingled in the lower 
part of the moving ice-sheet, up to bights of probably i,ooo to 
1,500 feet in the ice, and yielded to the streams of its final 
melting the shell-bearing gravel and sand of these high levels, 
as also of lower tracts down to the present coast lines. 
Jeffreys states that eleven species of the Moel Tryfan 
marine shells are arctic or northern, of which eight now range 
no farther south than the coasts of Norway, there living" at 
depths of from 10 to 20 fathoms; but that the other species (a 
large majority of the whole) are littoral or live in shallow 
water, all of these being probably now inhabitants of -the 
neighboring Carnarvon bay. 
With slight search I found in the sand and gravel on the 
northwest side of the Alexandra slate quarry, about 50 feet 
below the rock peak of Moel Tryfan, many minute particles of 
shells, from the size of a pinhead to an eighth or a quarter of 
an inch, and a few larger fragments, referred to three species, 
as determined by Prof. Kendall, namely Leda pernula Miiller, 
Tellina balthica L., and Fusus antiquus L., the last having its 
Pleistocene dextral form, distinguished thus from its uniformly 
sinistral Pliocene form. 
The Alexandra quarry, about 30 rods in diameter, having 
a depth of 75 feet or more below the lowest northeastern part 
of its rim and fully 150 feet below the high western part of the 
rim, is situated about an eighth of a mile east from the sum- 
mit of Moel Tr)'fan which rises some 40 feet above the high- 
est part of the brink of this quarry. The eastern half of the 
rim or brink has much till, to the depth of 10 to 15 feet, in- 
closing plentiful boulders up to five feet and rarely ten feet 
in diameter. Beneath this till on the southeast are a few feet 
of very irregularly stratified and contorted sand and gravel, 
