Yol. 63.] THE MOTION OF SUB-SURFACE WATER. 85 



In strata of uniform texture the porosity will vary inversely with 

 the pressure, and consequently with the depth beneath the surface, 

 until a point is reached at which the porosity has zero value. This 

 limit is styled by Prof. Yan Hise the base of the zone of 

 fracture, and is fixed by him, upon more or less empirical 

 deductions, as situate at a depth of about 6 miles below the surface. 

 Considering more commonplace conditions, the porosity will vary 

 with the past history of the strata and of the locality in which 

 they occur ; and it is most essential that the geological record should 

 be studied side by side with the flow-problem in every hydrological 

 investigation, although, strangely enough, this procedure seems to 

 have been the exception rather than the rule in the past, and the 

 subject seems to have beeu approached rather as a laboratory- 

 experiment, or a problem in hydraulics, with little or no reference 

 to the dynamical geological aspect. Thus, for instance, one theory 

 has been elaborated on the groundwork of experiments with sand- 

 filters ; but I have shown mathematically and experimentally, in 

 my other paper (24), that the porosity of sand depends almost 

 entirely upon the fortuity of its arrangement and previous physical 

 treatment, and it is at once evident that data obtained from experi- 

 ments with such sand are inapplicable in the case of, say, a sandstone 

 with inequalities of porosity in its cementing-material, and still 

 more so when that sandstone has been , faulted and folded or 

 subjected to infiltration. 



In a very interesting paper, C. C. Moore (12) has shown that, at 

 the Caldy-Grange fault at West Kirby, the porosity of the Keuper 

 Sandstone varies from 0*2256 of the total volume at a distance of 

 24 feet from the face of the fault, to 0*1650 at a distance of 3 inches 

 from the fault ; and, on the other side of the fault, the porosity of 

 the Bunter Sandstone varies from 0*1480 at a distance of 3 inches 

 from the face of the fault, to 0*255 at a distance of 25 feet from the 

 face, — or that, within a distance of only 49 feet, the porosity of the 

 strata varies from 1*00 to 0*74 and from 0*66 to 1*13. 



In the course of my own observations, I have noted similar 

 phenomena. Thus, in a flexure of folded red Peel Sandstone, 

 exposed on the shore of a little cove north of Whitestrand Bay, 

 near Peel, in the Isle of Man — a region which has been subjected 

 to pressures so severe that fragments of quartzite have been forced 

 into the main mass of slates of the district — the specimens col- 

 lected from the crown of the flexure had a porosity, expressed in 

 terms of the volume, of 0*161 ; while at a point of less curvature, 

 at a distance of only a few yards from the crown, the rock had a 

 porosity of but 0*034, or, roughly, 0*2 of that at the crown. 



The effect of an intrusive dyke of dark olive-green Tertiary 

 diabase in black Carboniferous Limestone, at Pool-y-vaash Bay, 

 Castletown (Isle of Man), is also shown in the following Table 

 (III, p. 86). 



