o(U PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
The direction of the ice movement is also indicated by the occur¬ 
rence of a number of boulders of the diabase upon the summit of^ 
Pine hill and in certain minor valleys or depressions between large 
outcrops of felsitic rock. According to Mr. T. H. Barnes, recently 
city engineer, similar boulders have been found in Medford, in 
several localities east of the dike, during excavation for sewers or 
water service. 
Denudation and erosion by the ice sheet. — Perhaps the most 
interesting indication of the direction of movement of the ice sheet 
is that furnished by the topography. In two localities this is most 
strikingly marked; at Pine hill in the Middlesex Fells, and at Pas¬ 
ture hill in Medford. Pine hill is, wdth the exception of a small and 
low portion of its northern end, a felsitic ridge cut across trans¬ 
versely by a large number of diabase dikes. The felsites are often 
very much broken up by several groups of intersecting joint planes. 
To the west lies the Medford dike, beyond which is more of the 
felsitic rock and then granite; the next hill to the east is a mass 
of granite. The ice sheet in its passage over the granite, felsite, 
and diabase has undoubtedly largely produced the topography 
as we see it to-day, due allowance being made for post-glacial 
changes of relatively small amount, such as the formation of talus 
slopes. The paucity of parting planes in the granite as compared 
with their abundance in the felsite has enabled the former to resist 
the glacial drag, while the latter everywhere along the line of the 
eastern contact has been pulled away, forming the present nearly 
vertical cliffs. 
The diabase dike within the felsitic rock was apparently even 
more easily torn out of its position by the ice. From the asymmet¬ 
rical topography of the valley and the disintegration of the diabase 
one would be inclined to infer that the rock had been weakened by 
decay during preglacial or possibly interglacial time. 
The diabase, in many places throughout its length, has undergone 
extensive disintegration', giving rise to loose sand and gravel of a 
deep brown color, in which lie rounded boulders of all sizes, 
produced by concentric weathering. These boulders show more or 
less concentric structure from without inward until a solid core of 
unaltered diabase is reached. The depth to which the decay has 
penetrated varies greatly; at Pasture hill in Medford it is in places 
as much as thirty feet, with almost complete disintegration of the 
