Shell-Bcari/ig Drift on Mod Try fan. — Upluun. 85 
which at the northeast increase ■ to a thickness of about ten 
feet. Proceeding thence westward along the north rim of the 
quarry, the till gradually thins to only two or three feet, while 
the sand and gravel continue from 10 to 15 feet thick upon the 
rising slope of the slate. Along the western and southwestern 
sides the drift capping the vertical quarry walls maintains a 
thickness of from 12 to 18 feet, and is almost wholly sand and 
gravel, often contorted in bedding, with occasional stones up 
to a foot in diameter, especially in the upper one to four feet, 
while the adjoining surface bears frequent boulders. 
Another quarry, of similar area and depth, has its northern 
brink only 40 feet south of the southern brink of the foregoing. 
Its slate is overlain around all its extent by 5 to 15 feet of 
drift, thickest at the northeast. This drift has mainly the 
stratification and other characters noted on the western half 
of the Alexandra quarry; but it includes, on the northeast, a 
thickness of several feet of overlying till. Superficial boulders 
are seen here and there on all the surrounding pasture land. 
From my studies of the till of drumlins in and adjoining 
Boston harbor, containing plentiful fragments of marine shells 
which represent 55 species, all now living on our coast, but 
some having mainly a southern range,* and from my discov- 
ery of similar shell fragments in the modified drift forming 
Cape Cod,t I conclude that the modified drift and worn and 
broken marine shells of Aloel Tryfan were supplied by the 
melting of the southern border of the principal British ice- 
sheet. In its maximum extent, that ice flow from the north 
abutted against local icefields that flowed outward from the 
Snowdonia mountain region. Their line or belt of junction 
appears to have crossed Moel Tryfan, and eastward to have 
passed over Fridd Bryn Mawr, which extends north from 
Moel EiHo along the west side of Llanberis and lake Padarn. 
On this Bryn Mawr, at the hight of about 1,000 feet above 
the sea, Ramsay found marine shell fragments in the drift. 
More explicitly to indicate the conditions which seem to 
me to have attended the deposition of the Moel Tryfan sand 
*Proc., Boston Soc. Nat. Hist, XXIV, 127-141, Dec, 1888 (also in 
Am. Jour. Sci., Ill, XXXVII, May, 1889). W. O. Crosby and H. O. 
Ballard, Am. Jour. Sci., Ill, XLVIII, 486-496, Dec, 1894. 
fAm. Naturalist, XIII, 489-502, 552-565, Aug. and Sept., 1879. 
