I 
1890.] 137 [Crosby. 
for no one supposes that glacial conditions obtained on our Atlan- 
tic seaboard in Miocene time. But when we pass to the Cretaceous 
clays of New Jersey the evidence is very conflicting. According 
to the report by Prof. George H. Cook on the clay deposits of New 
Jersey, published in 1878, the Raritan potters’ clay and the Wood- 
bridge and South Amboy fire clays contain nearly the normal pro- 
portion of water and very little free quartz or sand ; while the clays 
alternating with these show a close agreement in physical charac- 
ters and composition with the glacial clays of New England, afford- 
ing usually from 4 to 7 per cent, of water and 30 to nearly 80 per 
cent, of fine sand or rock-flour. 
Some sandy clays, especially where the sand is coarse, were prob- 
ably originally simply feldspathic sands, and the feldspar grains 
have subsequently suffered kaolinization. Before accepting the 
presence in a clay of a large proportion of rock-flour as an indu- 
bitable indication of a glacial origin, it is important to consider the 
question as to whether these materials may not under favorable 
conditions be deposited separately, even when they have been 
formed and transported in intimate association, and vice versa. 
Undoubtedly if, as has happened in most cases, the fine glacial silt 
is deposited in limited bodies of water — ponds, small lakes and 
the mouths of rivers — the flour and clay will each cover the entire 
area and be intimately commingled. But in larger basins, such as 
the great lakes, the conditions would undoubtedly be favorable for 
a partial if not a perfect separation of the flour and clay, the for- 
mer being deposited mainly in the marginal and the latter in the 
central parts of the basin. In the case, however, where the silt is 
discharged directly into the sea, it seems possible that the salt 
water may curdle and precipitate the main part of the clay as 
quickly as all but the coarsest flour, although we have seen that the 
clay from the mouth of the Mystic River is sensibly purer than that 
from West Cambridge on the same drainage system, but four miles 
farther inland. 
Assuming, as I think we may, that ordinary aqueous erosion 
would not, except perhaps under the most favorable conditions, 
yield so large a proportion of rock-flour as is found in the glacial 
clays, it is still conceivable that the accidents of erosion and trans- 
portation might bring together the components of a good glacial 
clay from wholly non-glacial sources. It appears, therefore, that 
while the glacial silts of every age must be, as a rule, if not always, 
highly siliceous, the converse is less true. In other words, a large 
