SO ANNUAL REPORT 
for cement manufacture we ought to know something in regard to the 
old or present river drainage which produced it. If the old river bed 
indicates a narrow stream with a rapid fall it would be idle to expect to 
find in its valley a fine grained clay. In most cases the old river valley 
is outlined by the gravel beds or terraces (see figure 6), so that its 
ee ee is ~ 
— —_——= a. 
oF - LT |. 
Y Wf); Y) 
Fig. 6. River Terraces. 
course can be followed with comparative certainty. If on the other hand 
the indications are that the stream was large and had a low velocity we 
have reason to expect that the fluctuation in the fineness of the clay ma- 
terial will be insignificant, and hence it is suitable as far as the physical 
characteristics are concerned. Again in a large river with somewhat 
greater velocity but with many curves and bends we should be encouraged 
-to expect fine grained clay along the pools formed by the bends in which 
the water is in a state of comparative quietness and hence the particles de- 
posited are very fine. 
River deltas are a source of a great deal of the very finest material, 
since there the velocity of the current is smallest and the chance to deposit 
the load carried by the water is greatest. 
It is important, therefore, that we should study and know the geologi- 
cal conditions under which the clays in question have been deposited. If 
found in an old river valley we should trace carefully all evidence in regard 
to the river now disappeared. If this were done much disappointment 
would be avoided. Frequently it has been found that a bed of very fine 
grained clay satisfactory in every respect degenerates at once into a coarse 
sandy clay or even gravel. It must be remembered that naturally the 
chemical composition will also vary with the introduction of coarse ma- 
terial. But if the problem of selecting suitable alluvial clays is attacked 
in this light it often is possible to find one corresponding to all the 
requirements. 
According to the character of the rock detritus carried along we 
have alluvial clays of all possible mineral compositions, from almost all 
sand to practically all clay. In general, however, the alluvial clays used so 
extensively in brick making range in composition between 25 to 60 per 
cent. clay substance, 5 to 25 per cent. feldspathic detritus and_o to 25 
per cent. of quartz. 
ae 
ae 
~ & 
fA 
Be 
