through rock on which they exert little or no dissolving effect,
instead of washing out fine particles, tend to dislodge any minute
grains of the stone that may not be firmly held by cement, and these
block up extremely fine and crooked pores in which the fluid is
passing.

Several tests indicated that this blocking up was largely near the
surface into which the fluid was passing. When this surface was ground
off, even 1/50 of an inch, the flow increased immediately nearly to
the original rate.

Reversing the flow also had the effect of increasing the rate, even
above that of any time previous.

With the moderate pressures used--from 2" to 80" of mercury--the
results show that the rate of flow, other things being equal, is
directly proportional to the pressure.

The porosity of rock is not always a criterion of its permeability; a
very fine grained marble, containing about 0.6 per cent. cell space,
transmitted water and oil more freely than a shale that would hold 4
per cent. of its bulk of water.

If the above conclusions hold on a large scale as on the small, they
may aid in explaining the diminished flow of oil wells. Not only will
the flow lessen from reduced gas pressure, but the passages in the
rock become less able to allow the oil to flow through.

The increase in flow following the explosion of large shots in a sand
rock may be due not only to fissuring of the rock, but to temporary