178 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
not be determined. In the widest portion exposed it does not exceed 
three feet in width. 
Another good example (Figure 4 A) is found in the coarse conglom- 
erate on the north side of Boylston Street just west of Thompsonville 
(Boston II, S 6-7). The lens is composed of sandstone indistinctly 
cross-bedded and is eighteen inches thick. At the eastern end it is 
sharply defined and its upper surface shows ripple-marks. The strike 
So vo 
052 LO Oo 
o O 
ESA 
co 
o 
Fic. 4.— A. Indistinctly cross-bedded lens of sandstone in conglomerate west 
of Thompsonville. 
B. Lenticular structure at Auburndale. 
C. Lenticular structure at Roxbury. 
is nearly east-west and the dip about 30° north. A few rods east 
another similar lens appears. 
In some localities lenticular structure occurs on a minute scale. 
Such an example is found in the ledges on the north side of Common- 
wealth Avenue in Auburndale (Boston II, C 31). Conglomerate, 
sandstone, and slate appear interbedded. The sandstone forms small 
lenticular masses in the slate as indicated in Figure 4, B. 
