STATE GEOLOGIST. 59 
amount of the finer particles is constantly increasing, which then are 
carried further down the stream till they either settle to the bottom, 
owing to the decreased stream velocity which cannot carry them any 
further or if fine enough till they have reached the still water of great 
lakes or oceans. 
It is evident, of course, that all this grinding must bring about a 
thorough mixing of the rock products. If the mountain creeks brought 
down, for instance, the fine white kaolin resulting from the decomposi- 
tion of feldspar together with ferrugineous minerals it is evident that — 
the clay substance is bound to be contaminated with the iron bearing 
materials. We can say, therefore, that sedimentary clays are always 
less pure than the white clay substance arising from the original rocks 
under favorable conditions. ‘The purest sedimentary clay on being burnt 
shows invariably a more yellow or reddish tinge than the washed pri- 
mary kaolin owing to the presence of more iron. But this, though of 
great importance to the potter, is not of any interest to the cement man- 
ufacturer, 
Since, as we have seen, the rock particles are carried along by a 
stream until the current velocity becomes too small when they settle 
to the bottom, we can also say that the greater the velocity of the current 
the larger will be the size of the particles deposited by it. The more 
rapid the flow of the river the coarser will be the sediment on its bottom. 
Very fine clay materials will hence be deposited only in oceans or lakes 
where the water is still. But since the velocity of a river varies we find 
that sometimes coarse materials alternate with fine; even at the same 
point in a river coarser material may be deposited in the middle of the 
stream while in a bend or pool, the water being comparatively quiet, fine 
clay settles out. We cannot, therefore, expect to find uniform deposits 
of clay formed by smaller rivers and creeks, but must look for them 
in beds of large rivers, lakes and oceans. 
Water deposition not only grades materials according to their size, 
but also according to their specific gravity. This is due, of,course, to 
the fact that heavy particles settle first, followed by the other materials 
in proportion to their specific gravity. Similar particles .of the same 
kind will hence be deposited together in horizontal layers. We must 
keep in mind this grading process of nature in order to understand why 
shales deposited in still water are so uniform in character, or why 
alluvial clays have such a varying composition. 
Alterative Influences to Which Clays are Subject.—We can readily 
see how clays may be deposited under various conditions. If the stream 
has carried along clay together with very fine ground quartz the re- 
sulting deposit will be a silicious clay, if with ground calcite or lime- 
stone we shall have a calcareous clay, or if with organic matter bitumin- 
