

ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 363 
gravel, sandstone, grit, conglomerate, and highly fissured 
anestones, ~~. ahr : : 
2. The thickness of the-entire succession of strata should 
be carefully measured, and the precise position in the series 
of any water-bearing beds should be ascertained. 
3. It is, further, most important that the average angle of 
dip should be determined, otherwise we shall be unable to 
estimate the depth from the surface at which the water- 
bearing beds may be expected to occur at any given point. 
4. The inclination of the strata must not be too great, for 
obvious reasons :—(@) Gently inclined strata have relatively 
broad outcrops, and therefore are in a position to absorb 
more rain than if they had been highly inclined or vertical, 
Thus, the outcrops of a series of strata, 100 feet thick, dipping 
at an angle of 1°, would be rather more than a mile in width ; 
while, if the angle of dip were 5°, the width of outcrop would 
be only 350 yards or thereabout—the width necessarily 
varying inversely as the dip; (6) With an inclination of only 
1, a stratum descends about 30 yards in a mile; but a dip of 
5° carries it down 147 yards in the same distance; while 
at angles of 10°, 20°, and 40’, the depths at which the stratum 
would occur would be about 300, 630, and 1490 yards 
respectively. It is obvious, therefore, that with a high dip 
a water-bearing bed must within a short distance descend to 
a greater depth than the engineer might consider it possible 
or desirable to bore. 
5. Let us suppose, however, that we have assured ourselves 
that the character of the strata is quite satisfactory, and that 
their inclination is equally favourable; we have still to 
ascertain whether the region is traversed by faults, dykes, 
or other discordant junctions which may serve as_ sub- 
terranean dams. It is quite obvious that, if any such 
obstructions occur, their position must influence the engineer 
in selecting a site for an artesian well. Jt would be hopeless 
to bore for water on the dip-side of a strike-fault or a dyke 
following approximately the same direction, whereas a boring 
put down on the rise-side would in most cases be successful. 
6. When the strata consist throughout of pervious beds— 
such as sandstones, highly cleft limestones, etc.—the chances 
of obtaining an artesian water-supply are much diminished, 
