Nov. 1, 1924 
447 
Dustfall of February 13, 1923 
Madison dustfalls. The latter also 
differ from the European dustfalls in 
the relative scarcity of calcium car¬ 
bonate. The only foreign dustfall 
which resembles the American material 
in this respect is that of New Zealand 
(Table IV, C.) 
In chemical composition the Madi¬ 
son dustfalls resemble the loess of the 
Mississippi Valley more closely than 
the dust from the African desert. 
This is shown in Table V. 
The loess from Mount Vernon, Iowa 
(Table V, E), and from Galena, Ill. 
( H ), contains considerably more lime 
carbonate than the Madison dust¬ 
falls (D, G). But in other respects the 
loess and the dust are strikingly 
similar. The dust of the 1923 fall (D) 
is more like the loess in chemical 
composition than is the dust of the 
1920 fall (G). 
MINERAL COMPOSITION 
It is impossible to determine micro¬ 
scopically the mineral components 
which form the finest “clay” particles 
of the dust, but those particles of the 
size of “very fine sand” (0.05 to 0.10 
mm.) can be recognized with little 
difficulty. In the dustfall of 1923 
such particles form one-third of the 
whole dust, by weight, and therefore 
give an approximate idea of the 
mineral composition of the whole, 
since the particles of different sizes 
are not radically different in composi¬ 
tion. A microscopic examination of 
the “very fine sand” disclosed the 
presence of very abundant quartz 
grains, a few feldspar grains, rather 
common limonite aggregates and grains 
thoroughly stained with limonite (which 
are hardly distinguishable from brown 
hydrocarbon fragments), and very 
minor amounts of hornblende, chlorite, 
carbonate, garnet, zircon, epidote, 
zoisite, apatite, tourmaline, hematite 
and magnetite. The feldspar is chiefly 
microcline with some albite. Most of 
the material is clear and readily 
identified except for that which is 
stained by limonite. 
It is possible to calculate the approx¬ 
imate mineral composition from the 
gross chemical composition by making 
certain assumptions similar to those 
made in calculating the mineral com¬ 
position of igneous rocks. It is as¬ 
sumed that all the soda is in albite 
feldspar, all the potassium is in ortho- 
clase (or microcline) feldspar, all the 
phosphoric acid is in apatite, all the 
titanic acid is in ilmenite, all the 
magnesia is in chlorite (in case of 
insufficient alumina the magnesia may 
be assigned to enstatite), all the lime 
remaining after forming apatite and 
calcite is in anorthite feldspar, all the 
alumina remaining after forming feld¬ 
spars and chlorite is in kaolinite, and 
all the silica remaining after forming 
all these silicates is in quartz. These 
assumptions are not all true, but they 
represent approximations and possible 
(even if not actual) combinations of 
the oxides into minerals. They furnish 
a useful means of comparing analyses, 
especially when the underlying as¬ 
sumptions are based so far as possible 
on determination of mineral compo¬ 
sition by microscopic study. The 
results of such computations so far as 
they relate to dustfalls, are given in 
Table VI. 
Table VI shows even more plainly 
than Table IV that the Madison dust¬ 
falls are exceptionally rich in quartz; 
they contain about as much feldspar 
as the New Zealand and one of the 
Naples dustfalls. The tenor of calcite 
and hematite is much lower than in the 
European dusts. 
Table VII shows the degree of simi¬ 
larity between the mineral composition 
of the Madison dustfalls and that of 
loess of the Mississippi Valley. 
It is evident that while the Madison 
dustfalls contain two to four times as 
much free silica (quartz) as foreign 
dustfalls they contain about the same 
tenor as found in loess of the upper Mis¬ 
sissippi Valley. The 1920 dustfall con¬ 
tained more feldspar and less kaolin 
than the loess samples, but these 
differences do not recur in the 1923 
dustfall. Some samples of loess con¬ 
tain abundant carbonate, but others 
contain little or none, like the dustfalls. 
Dr. J. G. Dickson, of the Depart¬ 
ment of Plant Pathology of the Uni¬ 
versity of Wisconsin, collected and 
examined five samples of dust-bearing 
snow, to determine the number of 
plant disease spores present. The sam¬ 
ples were obtained in several locations 
4 miles south and west of Madison, Wis., 
on the bench land. A layer of snow- 
bearing dust was scraped up without 
taking the bulk of snow either above 
or below. This was melted, and 10 
cc. centrifuged. Spores were counted 
by means of a haemacytometer. The 
results are given in Table VIII. 
