58 ANNUAL REPORT 
agent, shows its influence everywhere, especially with reference to the 
transportation of materials, there are comparatively few localities that 
escape its action. The natural result, hence, must be that the de- 
composed material is carried away, ground up in creeks and rivers and 
deposited, the coarse in rapidly moving water, the finer in still water, 
and the finest is carried out into the ocean or lakes to form the great 
deposits of shales. In this great grinding and deposition process the 
various portions of rock debris are mixed more or less thoroughly and can 
be said to possess a certain homogeneity only when deposited in deep still 
water, producing our fine-grained shales. All clays deposited by water are 
called secondary clays and comprise the vast areas of clay which we 
know in geology, the primary clays being mofe accidental in nature and 
small in extent. Thus the eternal cycle goes on. The igneous rocks are 
decomposed, worn away, carried by the streams and deposited; the beds 
-of shale, built up slowly in this manner by movements of the earth’s crust, 
are exposed and are again worn away to be redeposited. 
The clays of this class differ from the primary clays in several 
respects. Owing to the fact that they have undergone transportation 
and mechanical abrasion they are bound to be finer grained than the 
first class of clays and hence plasticity, the characteristic property of 
clays, is much more fully developed, except, of course, when sand or 
similar non-plastic material is mixed with it on the way from the old 
rock to the place of sedimentation. The work done by the water upon 
the mixture of rock minerals may be placed into four functions: 
3 1. ‘Transportation. 
2. Grinding. 
Be Mixinse 
4. Grading by sedimentation. 
Transportation must, of course, take place if the primary rock is to 
be converted into a sedimentary one. The agent of. transportation is 
always water, gathering in the mountains from small and weak streams 
fed by glaciers or springs. As these grow in volume and rush down 
into the valleys they carry with them large pieces of rock which are left 
behind at the foot of the mountains owing to the decrease in the velocity 
of the stream. With the slower current smaller particles of rock are 
brought along till finally near the sea the solids carried by the rivers 
consist of the finest particles of mud. We see thus that the size of the 
rock. fragments transported by the water is a function of the current 
velocity. According to W.Hopkins,* the transporting power of water in- 
creases as the sixth power of the current velocity; thus if the velocity 
is doubled its motive power is increased 64 times. 
But as the rocks are carried along and rubbed against the stream 
bottom they are continually undergoing mechanical abrasion and the 
*Physical Geology, A. J. Jukes-Browne, p. 128. 
