WILSON: MEDFORD DIKE AREA. 
stood about one quarter of an inch above the diabase surface, and 
assuming that the ice had planed the quartz off level with the rest 
(as it has done in two other noted cases of larger masses of quartz 
in an exposure with a well striated surface), this would give a par¬ 
tial indication of the amount which may be removed from the sur¬ 
face in the al)sence of a soil cover. In this connection it should be 
remarked that the glaciated surfaces of the felsitic rock and of the 
granite have also undergone alteration and loss to such an extent 
as to obliterate or obscure practicallj^ all striae if such were ever 
present. In those places where the striae were observed on the dia¬ 
base there is evidence that there has been a covering of soil to pre¬ 
serve them. In the case of the large area at Pasture hill about three 
feet of soil have been removed from above it, and at one time a 
large elm was growing over the place. In the exceptional cases re¬ 
ferred to, the glacial striae are now distinct on comparatively soft 
rock. This soft rock in every case, as far as observed, lies in the lee 
of hard and distinctly fresh rock relative to the direction of ice 
movement, both being striated in one case; in other cases the hard 
rock has been removed, presumably for building purposes. If dur¬ 
ing glacial times there had existed a mantle of disintegrated rock 
extending iq^ward to an unknown distance, and beneath this cover 
masses of partially disintegrated rock suri-ounding cores of still 
fresh rock (Fig. 2), such as are now found, the ice sheet in passing 
over Avould remove much of the superficial cover. In cutting across • 
the partially disintegrated rocks at the lowpr level (Fig. 3), it would 
necessarily reveal hard materials in close contact with relatively softer 
rock, but still hard enough to receive striations. It seems probable 
