WILSON: MEDLORl) DIKE AREA. 
371 
causing the ice to override the ridge and the somewhat softer por¬ 
tions partially in its lee. The ice must have overridden also the por¬ 
tion of the ridge to the south which is now disintegrated and which 
presents a much longer slope to the northwest. Here, if we suppose 
disintegration to be post-glacial, we should have again to postulate 
differential decomposition since the rock at both ends of this ridge, 
as far as observable, is quite decayed. It would perhaps be well to 
note in this connection that at the foot of the rock exposure, just 
southwest of the Powder House, there is evidence of glaciation, so 
that the outcrop as seen to-day stood out above the general level of 
the slates and bore the brunt of at least the last ice advance. The 
steep and unglaciated wall further to the north was probably pro¬ 
duced by a plucking action of the ice which, splitting on the ridge, 
Avould have a slight lateral motion. 
Summary and conclusions. —^ There are a number of areas of the 
diabase showing glacial striae on rocks which are jointed, and which 
are quite hard and practically fresh within an inch of the surface; 
the disintegrated rock occupies in every case protected areas, and in 
places where the disintegration is greatest, it lacks that uniformity 
of distribution which one would expect, had it taken place, since the 
present topographic forms were developed ; the boulders which have 
been carried away from the dike, and the dike itself in other locali¬ 
ties, although found in positions favorable either to atmospheric 
action or to the action of ground waters, are alike undisintegrated. 
On the basis of these facts as studied in the field even in much more 
detail than can be given here, it seems probable that the disintegra¬ 
tion of the diabase took place, for the most j^art, before the last ice 
advance. The amount of post-glacial disintegration is best indicated 
by the amount of disintegration seen on the still obscm'ely glaciated 
surfaces in the bottom of the dike valley at the western base of Pine 
hill, between the edges of the quarries and the eastern contact wall. 
Relationship between topographic form and country roch. — \t is 
interesting to note the close relationship existing between the char¬ 
acter of the rocks and the topographic forms produced by glacial 
action. At the northern end of the dike the more or less disinte¬ 
grated diabase is flanked by a much harder granite. Glacial scour¬ 
ing has swept the dike away for a considerable depth, and a nearly 
vertical cliff of granite is left standing to mark the western contact. 
Further south where the same dike rock lies in a much jointed felsitic 
