140 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
——:— Glacial. The till of the Glacial period, as described by J. 
Geikie and by Crosby, excluding the embedded stones, consists of 
tough stony clay, composed of grains of all shapes and sizes down to 
impalpable flour; or, when the matrix is more scanty, of coarse earthy 
grit and sand. The matrix as a whole is highly silicious, containing 
as much as 60 to 65 per cent of sand and rock flour in addition to 
coarser, quartzose material. 
The matrix of fluvio-glacial conglomerates, as described by J. 
Geikie and by Penck, varies from true boulder clay to well water-worn 
sand and gravel; its characteristics are not otherwise described. 
Ice-rafted boulders may be dropped into a matrix of well-stratified 
argillaceous sediments containing marine or fresh-water organisms. 
This seems to have been the case with the boulder-beds of India and 
New South Wales, though the latter are described as sandy in places, 
and containing a small proportion of fragments of undecomposed 
feldspar, minute chips of black shale and small pieces of carbonized 
plants. The matrices of the glacial conglomerates in South Africa, 
Norway, and Iceland appear to be more like the boulder-clay of the 
last glacial period, since they consist of fine mud, grit, and sand, con- 
taining angular fragments of quartz and other minerals, without any 
definite arrangement. 
:— Crush. In erush-conglomerates the materials of the matrix 
would vary with the rocks involved in the movement and the size 
of the particles with the intensity of the crushing forces. No data 
with reference to the general character of such a matrix are at hand but 
it is probable that the component particles would be angular, unsorted 
and of varying size. 
Pebbles:-- Marine. The instances cited show that the pebbles of 
marine conglomerates are usually composed of the more durable 
materials though they often vary locally with the nature of the adja- 
cent rocks. As regards size, the pebbles may be either coarse or fine, 
but there is a tendency within certain limits toward a fair degree of 
uniformity. There is gradation upward to finer sediments, in the case 
of the transgressing shore line, and horizontally in the direetion in 
which the conglomerate wedges out. The pebbles are uniformly well 
rounded except where they are in relatively close proximity to the 
sources from which they were derived. Cross-bedding frequently 
occurs. It may be noted, however, that the Pottsville, which, in its 
upper portions at least, has been described as a round-pebble con- 
glomerate, becomes more irregular in its lower portions, where it 
sometimes forms poorly assorted or even unassorted pebble or boulder 
