96 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
well defined and fairly regular escarpment of crystalline and meta- 
morphic rocks which extends southwest from Malden and Arlington 
to Waltham. ‘Thence the boundary turns more nearly south toward 
Auburndale and the slate belt becomes interrupted or reduced to a 
narrow zone which passes southwest by Newton Lower Falls to 
Wellesley and South Natick. The conglomerate does not extend 
north of the Charles River, though a single outcrop of a slaty rock 
with a few scattered pebbles does appear in Watertown between Mount 
Auburn and Belmont Streets. The northern boundary of the con- 
glomerate extends from Allston and Brighton almost due west in a 
fairly uniform line to Auburndale. Thence it also passes southwest 
by Newton Lower Falls to Wellesley and South Natick. With it is 
associated melaphyr in discontinuous masses. The surface in this 
western area is largely covered with glacial deposits but occasional 
outcrops seem to indicate that the conglomerate series continues 
beneath. 
The Southwest Extension. On Crosby’s map the southwest exten- 
sion is represented as a narrow tapering tongue extending from Newton 
Lower Falls to South Natick, with an eastward prolongation to Charles 
River Village. Tilton's work in this area seems to show that the 
boundary is more irregular than that given by Crosby and that the 
sedimentary zone is wider and extends farther south. The southern- 
most outcrops of conglomerate occur half a mile northeast and south- 
east respectively of Farm Pond. On the accompanying map the 
boundaries are given essentially according to Tilton’s mapping, ex- 
cept that on the eastern side the line passes northeast, without bend- 
ing south to Dover, to the Charles River, about three-quarters of a 
mile west of Charles River Village. This change is made on account 
of the fact that the occurrence of granite on the north side of the Charles 
River just where the latter is crossed by the boundary line and also at 
a point about three-quarters of a mile west-southwest of Dover makes 
it probable that the low ridge in that vicinity is composed of granite. 
The eastward extension at Charles River Village is made to include 
a large mass of melaphyr and a breccia that may be igneous or sedi- 
mentary, or partly both. In places the breccia becomes conglomeratic 
but even where this is not true the rocks appear to possess a greater 
affinity to the rocks of the basin than to the felsite breccias. From 
Charles River Village the boundary line passes north around the 
western side of the Needham felsite area and thence east to the north 
of Highlandville, where it joins the southern boundary of the main 
conglomerate area. 
