ORIGIN AND DEVELOPMENT OF ORE-DEPOSITS. 167 



The force of gravity is plainly insufficient even in the case of 

 open fissures, since any of these which may exist are already 

 full — and under pressures coresponding with their depths. Still 

 less can gravity account for interstitial circulation, which appears 

 to be altogether independent of pressure.* It would appear 

 that canalicular circulation can only be set up by a local lessening 

 of pressure such as would be produced by an ascending con- 

 vection current — and the very same hypothesis combined with 

 what we know of capillarity and surface tension will account for 

 the interstitial circulation. Thus then, given a permeable rock — 

 whether the permeability be due to minute capillary fissures, or 

 to the cleavage planes of its constitiient minerals, or to the 

 boundaries of its separate particles — given on one side a supply 

 of fluid, and on the other a withdrawal of the fluid, and there 

 must be interstitial circulation.! It is this interstitial water in 

 rocks which is called " quarry water " by Daubree. 



Let us now trace a little in detail the progress of this inter- 

 stitial circulation on the large scale — and with this object in 

 view, we may disregard secondary fissures, anticlines and 

 synclines, variations of porosity and permeability in the beds, 

 and similar irregularities. These would give rise to local 

 irregularities in the rate of the circulation and in the courses of 

 particular streams of particles, but would not otherwise affect 

 it — so that for the purposes of this enquiry we may regard the 

 ''country-rock" as a homogeneous and porous mass. Bearing this 

 in mind, we will try to picture the course of the water particles 

 through the rocks under ordinary conditions. 



* The tendency of water to pass through very narrow channels in all 

 directions increases as the channels are narrowed — while the tendency of one wet 

 particle of rock to wet another lying next to it is so great as to be practically 

 irresistible. M. M. Jamiu and Daubree have shewn that moisture travels — e.g. 

 through a block of sandstone in spite of an opposing gaseous pressure equal to 

 that of many atmospheres, thus answering Gay Lussac's difficult question " How 

 does water find its way to the volcanic focus without being forced back by a 

 tension of vapour below that is capable of sustaining columns of thousands of 

 feet of lavas ?" See Phil. Mag. 1861, and Daubree's Geologie Experiment ale. 



t Daubree classes rocks as '' permeable " and impermeable but not in a 

 strictly accurate sense, since all rocks are to some extent permeable. The rate 

 of permeability as well as the capacity to hold and retain water varies enormously. 

 Thus, while eurite only holds of " quarry water " 0"07 per cent., the clay of Meu- 

 don holds between 24 and 25 per cent. — Daubree, Les eaux actuelles, p. 6. 



