121 
[Crosby. 
like nearly pure clay, and is carefully dried without washing. The 
small amount of solid matter, if any, still remaining in the water 
— the siplionate — is estimated by weighing the whole and then, 
while the water is thoroughly agitated, taking twenty per cent, of 
it as a sample, which is evaporated to dryness and the impalpable 
residue, if appreciable, weighed and multiplied by five. The sura 
of these ten products, when carefully dried, affords by comparison 
with the original dry weight, as already explained, a check upon 
the manipulation. The samples were divided into so many differ- 
ent products in order to determine more exactly the relative abun- 
dance of the various grades of coarse and fine material, but partly 
also as a matter of convenience in sifting, since it reduced the 
amount to be caught and washed on any one sieve. 
In attempting to classify these materials no difficulty is present- 
ed by the coarser grades, or those actually obtained by sifting, for 
examination shows that they are all clean and free from clay ; and 
the first three, or those obtained on the 4, 6, and 12 mesh screens, 
are, as indicated in the subjoined list, conveniently and naturally 
called gravel ; while the second three, or those obtained on the 20, 
40 and 60 mesh screens, are true sands. The coarsest material 
passing through the 60 mesh screen, consisting of particles hardly 
more than of an inch in diameter, is, perhaps, too fine to be 
properly classed as sand ; but it seems best to draw the line here 
between sand and rock-flour, and to class with the latter everything 
between the third or fine sand and true clay. The real physical 
distinction between the rock-flour and clay is the hard, gritty nature 
of the former, no matter how fine it may be. The till, after the 
large stones have been eliminated, consists, then, in regular order 
from coarse to fine, of gravel, sand, rock-flour and clay; and the 
specially important and significant feature of these analyses is the 
relative proportions of these four constituents which they reveal. 
The gravel and sand are easily separated and distinguished, but 
the rock-flour and clay present serious difficulty in a purely me- 
chanical analysis. Number seven, the residue of the first decant- 
ing, contains, when properly washed, only slight traces of clay, 
which may be safely neglected ; but number eight, the residue of 
the second decanting, entangles and holds an appreciable quantity 
of clay, and it is practically impossible to separate them by wash- 
ing, for the finest of the flour will go off in suspension with the 
heaviest of the clay ; while number nine, the abundant siphon 
