Crosby.] 
128 
[May 21, 
contrasted in the forms of the constituent masses as in composi- 
tion. 
Rock-flour and Clay . — The tendency towards pure quartz ob- 
served in the finer sands is still more marked in the rock-flour. 
The residue from the second decanting (No. 8), especially, appears 
under the microscope simply as an aggregation of beautifully clear 
and somewhat rounded grains of limpid quartz. The clay, which 
the chemical test reveals, appears as an almost inappreciable dust 
on the quartz grains, or as a slight turbidity if the sample is thrown 
into a small volume of water. Grains of feldspar are rarely ob- 
served, and material that could be even doubtfully referred to the 
slate, diorite and other rocks observed in the gravel are entirely want- 
ing ; but the black oxides of iron, perhaps on account of their brit- 
tleness, appear to be more abundant than in the sand. They are 
easily separated by washing, and the menaccanite, as in the sand, 
seems to predominate. The rock-flour is thus essentially quartz- 
flour with a distinct trace of iron oxide. In other words, only 
these hardest and most stable component minerals of the rocks 
contributing to the formation of the till in this region have, at the 
last, escaped chemical change or complete dissolution and survived 
the powerful abrasive actions chemically intact. The concentra- 
tion of the iron oxides in the arenaceous portion of the modified 
drift, as in the sands of modern rivers and shores, is commonly at- 
tributed to the sorting action of water ; but back of that we have 
the important fact that they are akin to quartz in both mechanical 
and chemical stability and have thus successfully resisted the 
forces which have reduced to clay nearly all the materials, except 
quartz, with which they were originally associated. 
These observations are in perfect accord with the results of 
Daubree’s experiments in the artificial trituration of granitic rocks. 
Even where feldspar was the predominant constituent of the gran- 
ite, the sand obtained by its thorough trituration consisted entirely 
of quartz, the feldspar and all other silicates having been broken 
up chemically as well as mechanically, and reduced to clay. The 
well established principle that disintegration favors decomposition 
is thus seen to be of vast importance, even under the sevefe climat- 
ic conditions of the glacial period. In the original ledges, as well 
as in the bowlders and coarser fragments or gravel derived from 
them, we have a heterogeneous assemblage of minerals, a great di- 
