terion 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
reversal of the pressure, the force of the explosive tending to drive
the oil back for an instant.

The large shots now used (up to 200 quarts, or say 660 pounds of
nitroglycerine) must exert some influence of this kind, especially
when held down by 500+- feet of liquid tamping. In the course of these
tests, it was noticed that fresh water has a more energetic
disintegrating action on the shales and clay than on salt water.

This may furnish a reason for the fact, noticed by the oil men, that
fresh water has a much more injurious effect than salt in clogging a
well. No oil-bearing sand rock is free from laminae of shale, and when
fresh water gets down into the sand, the water must, as the
experiments show, rapidly break up the shale, setting free fine
particles, which soon are driven along into the minute interstices of
the sand rock, plastering it up and injuring the well.--_Engineering
and Mining Journal._

* * * * *

THE GROTTO OF GARGAS.

The grotto of Gargas is located in Mount Tibiran about three hundred
yards above the level of the valley, and about two miles southeast of
the village of Aventignan. Access to it is easy, since a road made by
Mr. Borderes in 1884 allows carriages to reach its entrance.

This grotto is one of the most beautiful in the Pyrenees, and presents
to the visitor a succession of vast halls with roofs that are curved
like a dome, or are in the form of an ogive, or are as flat as a
ceiling. It is easy to explore these halls, for the floor is covered
with a thick stalagmitic stratum, and is not irregular as in the
majority of large caves.

[Illustration: FIG. 1.--SECTION OF THE GROTTO OF GARGAS.]

Upon entering through the iron gate at the mouth of the grotto, one
finds himself in Bear Hall, wherein a strange calcareous concretion
offers the form of th