WILSON: MEDFORD DIKE AREA. 
3G9 
material of the interior of the decomposition spheres of the dike it¬ 
self. Now, these boulders must have been plucked from the parent 
dike and in their transportation were more or less freed from all 
disintegrated material. When subsequently left upon the summit of 
Pine hill, or buried in the soil, they must all, in one place or the 
other, have been exposed to meteorological conditions similar to those 
of different localities along the parent dike. In fact, as regards 
those buried in the soil, the conditions seem even more favorable 
for disintegration than would be found on the summit of Pine hill 
below the frost line, where the soil waters would drain off verv 
* C' 
rapidly. 
At the present time, the area of most extensive disintegration of the 
diabase dike is found on Pasture hill. This disintegration, however, 
is confined to the higher portions of the hill. The eastern contact of 
the diabase with the rhyolite breccia lies a little to the east of Governor’s 
avenue. From the records (including a photograph) in the city engi¬ 
neer’s office at Medford, it has been ascertained that the diabase at the 
eastern foot of Pasture hill, and hence underlying Governor’s avenue, 
shows veiy little alteration and only a very small amount of decay 
along the joint planes, the joint blocks being yet distinctly rectangu¬ 
lar in cross section. Now if the disintegration of the main mass of 
the hill has taken place within postglacial times, one would natur¬ 
ally expect to find similar decay in the locality described, which 
occurs at a lower elevation and is yet above the level of permanent 
ground water, and hence under more favorable conditions for action 
bv humic acids and other substances in solution in the soil waters. 
%/ 
Moreover, it seems strange that disintegration, which has affected the 
western four-fifths of the width of the dike so greatly, should affect 
the eastern side so little. The greater disintegration at Pasture hill 
than at Pine hill has already been explained as probably favored by 
the greater amount of jointing and slickensiding which is here quite 
evident. 
In the outline of the topography of the dike, attention has been 
called to the prevalence of steeply graded slopes or nearly vertical cliffs 
facing towards the southeast, or in the direction of the ice movement 
as shown by the glacial striae, with gentler slopes to the northwest. 
In passing over the land that now forms Pasture hill, the ice sheet 
was forced upward by the harder felsitic ridges to the west. When 
the softer material of the hill itself was reached, the ice continued 
