MANSFIELD: ROXBURY CONGLOMERATE. 97 
The Southern Boundary. The same obscurity that prevails in the 
case of the northern boundary recurs at the southern. The slates 
that border the conglomerate mass on the south are apparently identical 
with those on the north and the same question as to age and strati- 
graphic position again arises. On the supposition that the slates are 
conformable with the conglomerates and overlie them, the southern 
boundary of the formation lies along the northern edge of the granite 
area that stretches almost due west from Quincy through Milton 
Center nearly to Hyde Park. There it bends southwest toward the 
Neponset River. Whether the boundary around the western end 
of the Blue Hill Range is continuous with the conglomerate forma- 
tion that lies to the south is not certainly known. The broad open 
valley of the Neponset River at this place and the occurrence near 
Green Lodge of an outcrop of dense silicious sandstone would seem 
to favor this view but the deep cover of drift and alluvium effectually 
prevents any definite tracing of the formation across from the northern 
to the southern area. The occurrence of slate patches on the granitic 
area near Quincy and of isolated outcrops of slate in Hyde Park, both 
said to be cut by granitic dikelets and stringers, renders the correlation 
of the slate with the conglomerate doubtful. Some of the slate at 
least appears to antedate the conglomerate but how large a proportion 
of the southern slate belt is included in the earlier formation cannot be 
determined on account of the heavy drift cover. Professor Crosby 
believes that all the earlier slate is confined to the Hyde Park locality 
(p, p- 41) and to the patches on the granite (n, p. 428). The main 
slate area he classes with the conglomerate series. On the accompany- 
ing map the slate belt has been represented as a unit approximately 
in accordance with Professor Crosby’s view. Eastward, according 
to Crosby (n, p. 439, 508), the boundary forms almost a direct line to 
Weymouth Back River. There it bends southeast toward a point 
half a mile south of Hingham Harbor. Farther east the boundary 
line coincides in general with Weir River between Hull and Cohasset. 
Westward the conglomerate appears at Dedham about a mile west 
of Readville. Thence the boundary bends northwest for about two 
miles, then turns east-northeast in an irregular line through the south- 
ern part of the Stony Brook Reservation and Hyde Park to Mattapan. 
From that point it again bends westward for about eight miles to 
Highlandville where it makes another eastward jog of a mile and 
a half before making the final turn west to join the eastern boundary 
of the southwest extension. 
Area. On the supposition that the slates of Somerville and the: 
: 
