370 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORt. 
across it, removing the upper portions, grinding down the harder 
masses, which now appear here and there with glaciated surfaces, 
overriding the softer 23art8, as the glaciers of the present day in 
Norway and in the Alps and elsewhere locally override gravel and 
clay beds. To the eastward, either the area was at the time nearly 
as low as it is now or the softer rocks in the area jjermitted of its 
being lowered much more rapidly than at Pasture hill. The pluck¬ 
ing action of the ice, acting on the disintegrated rock, has removed 
large masses both of diabase and other rocks. In some places, as at 
the Hall i-oad valley, material has been removed from the full width 
of the dike ; in other places, only from j^ortions of the eastern side. 
In every case the field relations show that the amount of residual 
material now present, whether disintegrated or not, is dependent 
upon its position with reference to protection from ice action by 
harder material, with the single exception of the outcrop of the dia¬ 
base at the Powder House in Somerville, which is much less disinte¬ 
grated. 
At Pine hill the plucking action of the ice has formed the deej^ 
valley to the west, removing portions of the dike, chiefly from the 
western side, so that now the eastern contact wall is found uj3on the 
side of the hill at least thirty feet higher than on the western side. 
The jointed condition of the felsitic rock has lent itself particularly 
to the jilucking action of the ice, to such an extent that the entire 
eastern side of the contact with the granite, so far as traced, is 
marked b}^ a valley with a very steep and in many places vertical 
western wall. 
Pov'der House area, — At the Pow^der House in Somerville, 
where a southward extension of this dike outcrops above the soft and 
much jointed slate, glacial striae are present upon the outcrop. For 
a little over half the extent of the ridge the stoss side is little al¬ 
tered. Small amounts of disintegrated material are to be found 
lodged in the crevices. The disintegration along the joint planes is 
small. Along the remaining ijortion of the ridge to the south, the 
rock, so far as surface evidence goes, is quite disintegrated. This 
ridge, then, is regarded as a mass of the rock which was not decom¬ 
posed at the time of the passage of the ice, and hence it was able to 
resist the ice action. Immediatel}^ to the north, where disintegration 
had gone deeper, the dike was swept away to the general level of 
the adjacent country, but here the harder rock has resisted the action, 
