THE ECONOMIC ASPECTS OF SOIL EROSION 
By Du X. S. Shalkr, 
Professor of Geology in Harvard University and Dean of (he Laurence 
Scimtific School 
II 
As the scattered drops of a shower gather into streams the 
water begins to act in an erosive way. If this gathering takes 
]»lace at the height of five thousand feet above the sea, each pound 
of the fluid has a store of energy of position, which we term five 
tliousand foot-pounds, which it is to expend on its path to the 
sea. When the rain comes down on highlands, the first stage 
of its journey to the sea is commonly made at high speed in tor- 
rents. In the torrential division of a river system we always find 
the surface cast into steep-sloped valle3’s. Generally the whole 
surface is composed of inclines ranging in declivitv' from five to 
thirty degrees or more, on which surfaces the soil, if it be present, 
is alwavs moving down the incline at a variable rate. At times 
(especially’ yvhen softened by’ the frosts of yvinter and filled yvith 
yvater) the earth on these steep hillsides slips in the manner of 
a landslide down to the bed of the stream. There it for a short 
time blocks the yvay of the torrent, but it is swiftly carried doyvn 
to the riy’er channels. Commonly' the moy’ements of these earth 
masses is in the manner of a glacier, sloyv, but steadfast, amount- 
ing to a few lines or a few inches a year; rarely to a foot or two 
in that length of time. So general is this movement that although 
not readily’ noted yvithout precise observations it maj’ b\’ accu- 
rate observations be determined on nearly all steep slo]>es which 
are covered yvith a thick layer of soil. During the movement 
the expansion of the earth yvater in times of frost, the enlarge- 
ment of the roots yvhich penetrate the soil, and in a measure the 
process of yvetting and dr^’ing the detritus operate to thrust the 
mass, the movement being in all cases in the direction in yvhich 
gravit}’ inclines it to go. Arriving at the margin of the torrent? 
the procession of the detritus is cut ayvay by the syviftly moving 
yvaters and sent on its yvay to the sea. 
As above indicated, this process of erosion by the slipping of 
the soil toyvard the torrent beds is characteristic of all steeply 
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