872 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
rock, while both are affected, it is the jointed rock which has suffered 
the less, and the dike forms the bottom of a valley. Proceeding still 
farther south to Pasture hill in Medford, we find rocks of nearly equal 
resistance, a felsitic breccia and the diabase. Here the steep wall is 
formed by the diabase. In the softer slates of the basin to the south, 
at the Powder House area, the diabase is the stronger of the two 
rocks, standing out well above the softer slates, and acting even as a 
buffer to protect those which lie in its lee. 
Siunniar}/ and conclusions. — The study of the glaciation and the 
topography of the area have led to the following general conclusions: 
(1) The general direction of the ice movement in this portion of 
the margin of the Boston Basin has been between S. 22'' E. and 8. 
40° E., changing more to the eastward as one proceeds south. (2) 
In the production of topographic forms by glacial ice, the disinte¬ 
gration of the diabase and the jointed structure of the felsitic rocks 
have been important controlling agents. (3) A series of topo¬ 
graphic forms shows the diabase to be progressively more salient 
from north to south in rocks which are successively less resistant; 
viz., granite, jointed felsite, felsite breccia, and calcareous slate. 
(4) The disintegration of the diabase took place before the last ice 
advance, and the decomposed material found to-day is for the most 
part merely residual. (5) The glaciated areas are regarded as 
portions where decay had not penetrated so deeply as in other parts, 
and these were consequently first exposed to the ice and served in a 
measure to protect from erosion the soft rock near them. 
In conclusion, the writer desires to express his thanks to Mr. John 
Woodbury, secretary of the Metropolitan Park Commission, for a 
topographic map of the Middlesex Fells; to Mr. T. Howard Barnes, 
city engineer of Medford, for maps and information from the 
records of the city engineer’s office at Medford ; to Mr. E. W. Bailey, 
city engineer of Somerville, for a contour map of the Powder 
House area (Tufts Park), Somerville ; and to Mr. J. B. Woodworth, 
Dr. A. S. Eakle, and Dr. T. A. Jaggar, Jr., for criticism and advice 
while this paper was in preparation. 
