142 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
——:— (Glacial. The materials of which glacial pebbles are com- 
posed are largely local but there may be a considerable percentage of 
stones that have traveled great distances. In the main the transported 
fragments consist of durable rock but sometimes large blocks of weaker 
material are carried far from their parent rock without being com- 
minuted by ice action. The size of glacial boulders or pebbles may 
vary from tiny fragments to ponderous erratic masses, weighing many 
tons. The latter, however, have been transported chiefly on the sur- 
face of the ice and have not been subjected to the grinding and crush- 
ing suffered by those fragments which have been pushed along over 
the surface beneath superincumbent ice. As a consequence there is 
generally a gradation in size in the pebbles of a glacial accumulation, 
the upper portions as a rule consisting of coarser materials. In shape 
glacial boulders are usually angular or subangular, and those which 
have been transported beneath the ice have one or more flat sides, 
rounded edges, and more or less snubbed ends. Perhaps the most 
distinctive feature of glacial pebbles is the beautiful polish and striation 
which the more fine grained and durable fragments receive, the 
markings running, on the whole, parallel to the long axis of the stone. 
When these materials are rehandled by intra- or extra-glacial waters 
or by the sea, the glacial markings are soon lost and the shapes of the 
pebbles become similar to those produced by fluviatile or other aque- 
ous agency. 
Another striking characteristic of glacial accumulations is the 
marked disorder of their component fragments. Masses of all sizes 
and shapes are huddled together in indiscriminate confusion. Some- 
times, however, there are included masses or pockets of stratified 
material, and in the reworked glacial deposits all stages of order are 
represented from complete heterogeneity to well-marked stratification. 
———:—Crush. The pebbles of erush-conglomerates vary with the 
nature of the rocks involved in the crushing. No data with reference 
to size are at hand but it is probable that there is considerable vari- 
ation. The shape is described as rounded or lenticular, sometimes 
showing the remains of crests or limbs of folds that have been formed 
into augen-structures. 
Color:—Marine. In the accounts of marine deposits above cited the 
conglomerates are mentioned as having a ferruginous matrix, and 
again as being interbedded with red and blue clay. In the case of 
the Pottsville, the lower portion is described as variable in color, 
bands of conglomerate alternating with washes of purple and olive 
mud or soft greenish sandstone. The upper portion, however, is light 
