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 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.]