124 BULLETIN! MUSEUM OF COMPARATIVE ZOÖLOGY. 
(3) A comparison of glacial silts with Miocene clays from Gay Head 
shows that while the former contain a high percentage of rock flour 
in comparison with clay, the latter are more argillaceous. On the 
other hand the evidence from the Cretaceous clays of New Jersey is 
conflicting. ' While therefore, “glacial silts of every age must be, as a 
rule, if not always, highly siliceous, the converse is less true,” (ibid., 
p. 136-137). 
The pebbles or stones in the boulder-clay are thus described by A. 
Geikie (p. 1311): “They are usually oblong, have one or more flat 
sides or ‘soles,’ are smoothed or polished and have their edges worn 
round. When they consist of fine-grained, enduring rock, they are 
almost invariably striated, the striae running on the whole with the 
long axis of the stone, though one set of scratches may be seen cross- 
ing and partially effacing another. . . . These markings are precisely 
similar to those on the solid rocks beneath the boulder-clay.” Sim- 
ilar features are noted by Stone (p. 25). 
The so-called “glacial gravels” are, strictly speaking, water-laid 
deposits, yet they are so intimately associated with glacial action that 
they may appropriately be treated in this section. ‘They are extensively 
developed in northwestern Europe and northeastern North America 
and have formed the basis of a considerable literature, from which a 
few citations will suffice to give an idea of their general character. 
In the discussion of eskers, J. Geikie states (p. 169) that “the steeper 
ridges are composed chiefly of gravel, generally coarse with little or no 
trace of bedding. In many places they may consist of tumultuous 
heaps of coarse gravel, shingle and water-worn boulders or of agglomer- 
ates of large blocks and angular and subangular debris mixed with 
earthy grit and sand. The abrupt embankments on the other hand 
are usually built of finer gravel and sand and are often beautifully 
bedded.” There is a frequent passage (ibid., p. 170) into true mo- 
rainic material; large and small angular fragments of rock occur in 
the heart of the water-worn gravel. The bedding is confused and 
the stones have a less rounded aspect in the upper part. 
Terraces are described by J. Geikie (ibid., p. 175) as deposits not 
always well water worn. ‘Sometimes indeed they consist only of ang- 
ular and subangular stones and a kind of earth or earthy sand and 
clay. 
Kames (ibid., p. 183) are usually stratified and in some fine grained 
accumulations there are beautiful examples of false bedding. In 
many cases coarser heaps of shingle are piled in confusion without 
traces of stratification. It is remarkable that the gravel stones whether 
