Crosby.] 310 _  [November5 
and at their poles have often become repositories of segregated min- 
erals, chiefly crystalline quartz; (4) that some of the :pebbles in the 
conglomerate at Chestnut Hill Reservoir seem to have been com- 
pressed, or flattened without fracture, 1.e., to have been truly plastic. 
I have not seen the flattened pebbles to which Prof. Niles refers, 
but the pebbles which I have observed at another point in the same 
belt of conglomerate, and to which I wish to call the attention of the 
Society, have outlines that, as it seems to me, can be satisfactorily 
explained only on the supposition that the material has been slightly 
plastic. The precise locality of these pebbles is the ledge of pud- 
ding stone on the north side of North Beacon Street, a few rods west 
of Everett Street, in the Brighton district of Boston. The pudding 
stone, at this point, is inclined to the north at an angle of about thirty 
degrees; and the exposure is very satisfactory, having an area nearly 
one hundred feet square, and presenting on the north a straight, 
smooth slope of rock parallel with the bedding. The conglomerate 
includes many limited layers of slate and sandstone. These are most 
numerous toward the north, and the ledge is terminated in that direc- 
tion by a bed of sandstone several feet thick. The slaty layers are 
mostly quite small,— a few inches to several feet in diameter,— and 
often present the general aspect of pebbles, though they are probably 
mainly due to irregular sedimentation. They all show a well-marked 
cleavage in planes parallel with the bedding. 
The pebbles of the conglomerate, which, with rare exceptions, are 
well rounded, are of all sizes up to a foot or more in diameter; the 
average diameter, however, not exceeding two or three inches. They 
are principally composed of a very firm, whitish, fine-grained quartz- 
ite, several varieties of petrosilex, and granite. The quartzite peb- 
bles predominate, and it is in this class chiefly that the outlines appear 
to be distorted. The distortion of the pebbles is most noticeable in 
those portions of the conglomerate where they are most thickly 
placed, and I have failed to detect any deformation where the pebbles 
appeared to be entirely isolated in the paste; actual contact of peb- 
ble with pebble seeming to be essential to the distortion, though I am 
possibly in error here. 
The general outlines of the pebbles are rarely extensively altered; 
i.e., as arule there is no sensible flattening of the pebble as a whole, 
but we find instead indentations and local flattenings. The indenta- 
tions are usually quite shallow, and, since the quartzite is a rock little 
affected by weathering, they are most favorably exposed on the 
