MANSFIELD: ROXBURY CONGLOMERATE. 173 
the northern border of the granite west of Quincy. In Wellesley 
and South Natick also the conglomerates and grits in the vicinity of 
granitic or felsitic areas are highly arkosic or felsitic. 
——:—Conglomerate. By reference to the map (Plate 7) it will be 
seen that the conglomerate of the Boston Basin is separated into three 
elongated areas by nearly east-west bands of slate. The conglomerate 
masses thus separated are relatively broad at their western ends and 
diminish in width eastward until they disappear beneath the slate or 
terminate at the sea. In addition to these larger areas two important 
isolated smaller areas of conglomerate occur in the southeastern part 
of the basin at Hingham and Nantasket and an irregular tongue of 
uncertain dimensions projects southwest from the vicinity of Newton 
Lower Falls through Wellesley and South Natick. The major con- 
glomerate bands are more or less interrupted toward the west by masses 
“of melaphyr and the two southern bands are split at their western 
ends by masses of the underlying felsite or granite that project eastward 
from Needham and Hyde Park. 
——:—Sandstone and Grit. Sandstone and grit are not of sufficient 
abundance to form units that can be mapped but they are important 
and sometimes conspicuous members of the conglomerate series. They 
are interbedded with the conglomerate and shade into it. While they 
Sometimes occur as indefinite zones or streaks within the conglom- 
erate toward the central parts of the great conglomerate bands, they 
appear more frequently along the northern zone of the northern band 
and along the north and south borders of the other bands. They are 
also more abundant toward the east than toward the west. E 
——:—Slate. Slate bands extend westward between the conglom- 
erate areas toward the western border of the basin. Eastward they 
broaden and appear to merge into the great mass of slate that is 
believed to underlie Boston Harbor. On the north and on the south 
of the general conglomerate mass extensive areas of slate occur which 
appear to merge around the eastern ends of the conglomerate bands 
with the narrow slate zones. A narrow slate band accompanies the 
Southwest extension of the conglomerate through Wellesley and South 
Natick. At its northeastern end direct connection with the broad slate 
area of Somerville and Cambridge is not observed but the evidence of 
the few available outcrops seems to show that such connection exists. 
Along the borders of the conglomerate areas the slate is interbedded 
with conglomerate and sandstone, into which it appears to pass con- 
formably. So far as field evidence goes the slates thus interbedded 
and interbanded with the conglomerate appear to be identical with the 
