MANSFIELD: ROXBURY CONGLOMERATE. 201 
:— The Dorchester-Roslindale Slate. A relatively narrow belt of 
slate extends from Roslindale nearly due east through Franklin Field 
and Dorchester to the coast. Outcrops are few. Topographically the 
belt is marked by a broad valley which is especially well defined near 
Roslindale and is utilized by the Dedham Branch of the New York, 
New Haven and Hartford Railroad. The structure of this belt has 
long been regarded by Crosby as a closely folded, somewhat unsym- 
metrical syncline, because the conglomerate both north and south 
Seems to dip beneath it (Crosby, g, p. 9). There has been, however, 
Some difference of opinion with regard to the matter. Dodge, for 
example, stated in 1881 that he did not know of a single fact support- 
ing the supposition that the West Roxbury-Dorchester slate lies in a 
syncline (Dodge, b, p. 209). Nevertheless the writer is inclined to 
accept Crosby’s view of the structure, for at the corner of Bellevue 
Avenue and West Street in Roslindale a beautifully glaciated ledge 
of slate that has been rather recently uncovered shows what appears 
to be a faulted synclinal fold. The rock at this place is rather sandy 
and in places well banded. The entire length of the exposure from 
north to south is about thirty-five or forty feet. Variations in texture 
occur along this line. For the first twenty feet from the north the 
tock is sandy, then follow five or six feet of alternate sandy and slaty 
layers, with some development of cross-bedding, and finally ten feet 
of sandy slate. The northern limb of the fold strikes N 67° E and 
has a dip of 50° south. As the fault is approached the strike becomes 
a little more easterly and the dip somewhat steeper. The fault is 
marked by a brecciated zone three to six inches wide which strikes 
57° E and has a vertical or steep southerly dip. The southern 
limb of the fold is parallel in strike and dip to the fault zone. The 
Strata of the two limbs do not correspond and the evidence of inver- 
Sion of the layers of the southern limb is not particularly clear. It 
is possible that the slate belt may be merely a slaty member of the 
Conglomerate series which has been interrupted by a strike fault, and 
that the appearance of folding may be illusory. But this would 
Involve a greater thickness and a somewhat different arrangement 
of strata on the south side of the great Savin Hill-Brookline anticline 
than appears on the north side. Moreover, the conglomerate to the 
South of the slate belt grows coarser in the vicinity of Mattapan and 
Hyde Park in much the same way that the similar change occurs 
northward toward Brookline. Thus the suggestion is strong that 
the strata south of the slate belt appear in descending order in the 
Same way that they do northward, that the slates occupy a strati- 
