210 
early summer rain is more likely to fall gently, 
and at night. In the southwestern semi-arid 
late summer rain area (7. e., Arizona, New 
Mexico and adjacent territory) the railroads 
have been put to considerable expense to build 
dikes to protect their tracks from sheet-floods 
following summer showers, while in northern 
Nevada and Utah, where the total precipitation 
is about the same, but its seasonal distribution 
different, no such precautions seem to be nec- 
essary. Floods of the ordinary type, caused 
by overflowing rivers, are much more frequent 
and destructive in the region of early summer 
rains, however. 
If there was such a thing as a soil fertility 
map of the United States it would be seen at 
once that the most fertile soils are in the 
region where there is more rain in early sum- 
mer than in late summer, and vice versa. 
Considering texture only (for we now have 
much more complete data on that than on 
chemical composition), we can ascertain from 
published soil surveys that silt loam—which is 
usually considerably above the average in fer- 
tility—is one of the commonest types of soil 
throughout the Mississippi valley, whether it 
is derived from weathered Paleozoic rocks, as 
in Tennessee, from glacial drift, from glacial 
lake deposits, as in North Dakota, or is of 
wolian origin, as the loess of Arkansas and 
Mississippi is supposed to be. Clay loam and 
stony loam are other common types in the same 
area, nearly or quite as fertile as the silt loam, 
while sand is chiefly confined to the banks of 
streams. ‘The black prairies of Alabama and 
Texas, characterized by very fertile caleareous 
clays, are both in the area of early summer 
excess of rain, although when semi-annual 
percentages only were considered, as hereto- 
fore, the line of equal summer and winter 
rains passed between them. One would nat- 
urally suppose the flood-plain of the Missis- 
sippi below the mouth of the-Ohio to be one 
of the most fertile tracts in the world; and so 
it is where it has more rain in early summer 
than in late summer, and most of the farmers 
in that portion use no commercial fertilizer 
whatever. But in several parishes below New 
Orleans, where the late summer rain is in ex- 
SCIENCE 
[N. 8. Vou. XLVIII. No. 1235 
cess, the average expenditure for fertilizers in 
1909 was over a dollar per acre of improved 
land! (For the whole United States at the 
same period the average was about 24 cents.)® 
The regions of heavy late summer rain are 
characterized by poor sandy soils, classed by 
the U. S. Bureau of Soils as sand, sand-hill, 
sandy loam, fine sandy loam, ete., and silt 
loam and clay are comparatively scarce. The 
sandiest extreme is in peninsular Florida, but 
northern Michigan, Cape Cod, and southern 
Texas are also notoriously sandy. Swamps 
too are about as prevalent in northeastern 
Minnesota and northern Michigan as they are 
in Florida. Although the late summer rain 
area covers only about one fourth of the 
United States and produces considerably less 
than one fourth of the crops, it uses at least 
three fourths of the commercial fertilizer; the 
average annual expenditure for that in some 
of the southeastern counties of Florida at the 
time of the Thirteenth Census being about $30 
per acre? And even in California there is 
considerable fertilizer used in the southeastern 
portion. 
In all these eastern sandy and swampy areas 
the streams carry very little sediment and do 
not fluctuate much. In the corresponding 
parts of the Southwest there is not so much 
sand, but the type of soil known as adobe (and 
used extensively for building material) is very 
characteristic. 
The distribution of vegetation types is of 
eourse correlated with the soil to a consider- 
able extent. Except in the Northwest the for- 
ests of the early summer rain area are com- 
posed almost wholly of deciduous trees, and 
there‘are vast areas of prairie; while conifers, 
especially pines, predominate in northern Wis- 
consin, Cape Cod, southern New Jersey, 
Florida, and many other places where there 
is more rain in late summer. The most ex- 
tensive pine forests in North America are said 
by Professor Sargent!! to be those of western 
9In this connection see ScimeNcg, II., 42, 500- 
503, October 8, 1915. 
10 See Geog. Review, 4, 225, September, 1917. 
11‘‘Manual of the Trees of North America’’ 
(1905), p. 16. » 
