176 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
materials seem to be deposited together, but mainly upon the relative 
proportions of these two. 
From the studies of the previous chapter the shape of the pebbles 
of the conglomerate has been shown to be subangular to rounded, 
while the finer fragments are generally subangular to angular. ‘These 
features are very clearly shown in the field. The smaller pebbles, 
which usually attain the more rounded forms, are still in many cases 
subangular, while the large masses one foot or more in diameter are 
subangular or even angular. 
Bedding.— The bedding of the conglomerate is best displayed along 
the borders of the several areas where frequent alternations with the 
finer sediments occur. In the central zones of the conglomerate areas, 
where finer material is scattered among the pebbles with little or no 
definite arrangement, it is difficult or impossible to determine the bed- 
ding. The attitude of the pebbles in the rock helps but little for they 
are often so huddled together that the flat sides do not seem to lie in 
any particular plane. In the Newton-Brookline area, for example, the 
bending is not easily determined while farther north or south near . 
the borders of the slate belts the attitude of the rocks is fairly clear. 
The pell-mell arrangement of the pebbles in the coarser conglomerates 
and the scattering of large pebbles and boulders through finer sedi- 
ments indicate rapid and irregular deposition. Two general forms 
of bedding are here distinguished: bands, where the upper and lower 
boundaries of the several strata are parallel or roughly so, and lenses, 
where the upper and lower surfaces of a layer converge. Sometimes 
only a portion of a lens may be seen, in the form of a wedge. In such 
cases the corresponding portion may be concealed by displacement 
or by some covering or perhaps it may have been eroded away. 
:— Bands. In the Boston Basin banding is the prevalent type 
of bedding and is the feature commonly relied upon for the determin- 
ation of the attitude of the strata. It is by no means a constant char- 
acteristic, however. Of the 283 outcrops of conglomerate, sandstone, 
and slate, visited by the writer and described in his field notes, only 
101 showed banding. Most of the observations recorded occur on 
the Boston sheet. In the western strip (Boston II and III) eighteen 
out of seventy-one outcrops (about 25 per cent) show banding. In 
the central strip (Boston V and VI) fifty of the 131 outcrops noted 
(about 38 per cent) are banded, while in the eastern strip (Boston 
IX, including the adjacent areas of Boston Bay III and VI) thirty- 
eight out of seventy outcrops (about 54 per cent) display similar fea- 
tures. The figures given are not absolute but they serve to show a 
