No. 2. —THE MEDFORD DIKE AREA. 
BY ALFRED W. G. WILSON. 
That portion of the rim of the Boston Basin which may be clesisf- 
natecl the Medford Dike Area, comprises a narrow belt extending 
north-eastward along the line of the Medford diabase dike from 
Medford city, on the Mystic river, for a distance of about two miles. 
The present paper presents the results of a detailed field study of a 
jjortion of the complex series of igneous rocks which form the 
northern rim of the Basin. The jd rimary object of the work was to 
prepare a detailed map of the northern end of the large dike of dia¬ 
base which occurs in IMedford and in Somerville, Mass. The work 
necessarily required the examination of a narrow belt upon either 
side of the great dike. Evidence for the interpretation of the rela¬ 
tions of some of the rocks is either incomplete or wanting within 
the area. The facts for a final interpretation will probably be 
determined when the detailed studies now being carried on in the 
Middlesex Fells, under the direction of Dr. T. A. Jaggar, Jr., are 
completed. The latter part of this paper presents some studies of 
the topographic features of the area. 
Areal, Petrographic, and Dynamic Geology. 
Rhyolite breccia. — The most southern member of the series of 
rocks, outcropping in this area on both sides of Pasture hill, is a 
rhyolitic breccia. On freshly fractured surfaces the colors of this 
breccia vary from dull gray to pale grayish pink or greenish. In 
thin section, the breccia shows traces of flow structure in the ground 
mass between the fragments. At one locality, west of Hillside 
avenue, a large irregular mass of soft, dark, almost black, shale-like 
rock occurs, included within the rhyolite. Under the microscope 
the nature of this inclusion is indeterminate, except that it carries 
a large amount of hematite. 
SecUrnentary rocks. — North of the breccia, on the west side of 
the Medford diabase dike at the foot of Pasture hill, is a bed of 
conglomerate apparently dipping 30° N. and striking approxi¬ 
mately east and west. The deposit is, in this locality, about fifty 
