MANSFIELD: ROXBURY CONGLOMERATE. 189 
clearly shown in the coarse conglomerate of the Narragansett Basin 
as in the other basins. The large pebbles at Purgatory have been so 
distorted that their original shape is not well seen. From their pres- 
ent appearance, however, it may be inferred that they were originally 
‘subangular to rounded. Thus it is probably true in this basin as in 
the others that the larger pebbles are more irregular in shape than the 
smaller pebbles. 
Bedding. The writers observations in the Narragansett Basin 
have not been sufficiently numerous to furnish numerical data with 
reference to the features of bedding. The evidence at hand, how- 
ever, goes to show that the bedding is ordinarily well indicated, except 
in the case of the coarsest conglomerates. 
——:—Bands. Banding is again the prevalent type of bedding and 
displays in general the characteristics noted in the Boston and Norfolk 
Basins. In the coarser textured rocks it is likely to be more or less 
indefinite with a tendency to the development of streaks. In the 
finer sediments the bands are usually well defined. 
——:— Lenses. Lenses are of relatively infrequent occurrence. 
They present the same characteristics already noted in the Boston 
and Norfolk Basins. 
——:—Cross-bedding. Cross-bedding occurs in some of the sand- 
stones and grits but only occasionally. The best example known to 
the writer is found in the ledges at the Thatcher Road bridge south of 
the Attleboro station. ‘There both banding and cross-bedding are 
beautifully exhibited on the natural polished surface of the ledge. ‘The 
attitude of the cross-bedding indicates that the upper surface of the 
beds is on the north. 
——:— Ripple-marks. Some of the finer sediments have ripple- 
marked surfaces which bear organic markings or the imprints of rain- 
drops. At the quarry half a mile south of the junction of Thatcher 
and County Roads in Attleboro an excellent exposure of micaceous 
shaly sandstone displays all these features. ‘The ripple-marked sur- 
faces, now nearly vertical, are marked with raindrop impressions, 
worm-trails, and rush-like imprints of stems or leaves. In the railway 
cut a mile north-northeast of the station at Perrins in Seekonk 
the dark gray, micaceous, shaly sandstone bears obscure casts, possi- 
bly of organic origin, while in the same section more carbonaceous 
and shaly layers contain plant stems. 
——:— Local Unconformities. Local unconformities, inferred in the 
other basins from evidence derived from the pebbles, are clearly exhib- 
ited in some parts of the Narragansett Basin. The ledge at Thatcher 
