1890 .] 
123 
[Crosby . 
the average percentages not only of the ten parts into which each 
original sample was divided, but also of the four distinct kinds of 
detritus composing the till : — ■ 
TABLE I. 
c (1) Coarse, 17.08 \ 
Gravels (2) Medium, 2.99 ? 24.90 
‘ (3) Fine, 4.83 ^ 
r (4) Coarse, 3.33 j 
Sands (5) Medium, 9.25/19.51 
* (6) Fine, 6.93 
f (7) Coarse, 12.80 ^ 
Rock-flour j (8) Medium, 6.52 i 43.86 
j (9) Fine, . . 
(_ (10) Superfine, 
. . 24.11 j 
. . 0.30 j 
99.94 
The eighth, ninth and tenth products, as already explained, are 
necessarily divided, the finest portion of each, determined by ig- 
nition, being classed as clay and the remainder as rock-flour. It 
appears from these data that the normal till of the Boston Basin 
is composed, after the larger stones are excluded, of about 25 per 
cent., or one-fourth, of coarse material which may be classed as 
gravel ; about 20 per cent., or one-fifth, of sand ; 40 to 45 per cent., 
of extremely fine sand or rock-flour, and less than 12 per cent, of 
clay. The essentially normal character of these averages is evi- 
dent from the general agreement of the individual analyses, as 
shown in the following table, comparatively few of the determina- 
tions varying greatly from the mean : — 
