MANSFIELD: ROXBURY CONGLOMERATE. 177 
steady increase eastward in frequency of occurrence of ‘well-defined 
bedding, corresponding to the increase in the proportion of finer 
materials in the conglomerate. A similar, though not so pro- 
nounced, increase takes place along a north-south line with refer- 
ence to the slate belts. In Boston II four of the eight occurrences 
of banding are within a mile of the northern slate area. In Boston 
III, seven of the ten banded outcrops observed lie within a mile or 
a mile and a half of the Chestnut Hill-Newton Upper Falls slate belt. 
In Boston V the case is not so clear. Only eight of the thirty-one out- 
crops described are banded but these appear to have a fairly uniform 
distribution with reference to a north-south line. Boston VI is crossed 
by the Chestnut Hill slate belt on the north, the Roslindale slate belt 
in the center and the Neponset slate in the southeast. The forty- 
two banded outcrops recorded appear on the whole to be symmetri- 
cally arranged with reference to the slate belts, so that the coarser 
conglomerates with less clearly defined bedding occupy axial positions 
with reference to the conglomerate zones. In Boston IX and in Bos- 
ton Bay III and VI the outcrops of banded conglomerate are also in 
proximity to the slate areas. 
The bands by which the conglomerate is marked may be of coarser 
conglomerate in fine or vice versa, or of grit, sandstone, or shale. Their 
outlines are generally sharply and definitely marked. Sometimes, 
however, they are but poorly indicated and the bands become ill-de- 
fined zones of texture finer or coarser than the enclosing rock. If 
such a band is narrow it appears as a mere streak. The occurrence 
of these indefinite zones or streaks is relatively infrequent. Of the 
283 outcrops of the Roxbury series recorded by the writer only four- 
teen (about 5 per cent) were characterized by such zones. 
:—Lenses. The occurrence of lenses in the Roxbury Conglom- 
erate is much less frequent than might be supposed. ‘The writer's field 
notes show only twelve localities. Probably the figures in this case 
and in that of the streaks above mentioned are too low, but the fact 
remains that both of these types of bedding are relatively uncommon. 
The lenses are found at several localities in Boston I, HI, VI, and IX 
and Boston Bay VI. The largest one observed occupies the lower 
part of a ledge exposed for fifty feet on the grounds of the Hospital 
for Women and Children in Jamaica Plain (Boston VI, UV 7). The 
lens consists of angular fragments from half an inch to one and one- 
half inches in length and has a gentle southeasterly dip. At the 
west it terminates a few feet from the end of the outcrop and east- 
ward it passes beneath the soil cover so that its true dimensions can- 
