404 Rev. A. Irving—Water Supply from Bagshot Sands. 
as to slide easily over each other, they have altered their lateral’ 
dimensions in that way without any actual rupture. Where, on the 
other hand, the rocks were thickly bedded, or otherwise so constituted 
that they could not so readily adapt their dimensions to the larger 
space by lateral extension, they have given way at intervals; and 
they would give way more readily along the planes of weakened 
cohesion formed by the “bate” than elsewhere—more especially 
when, as must very frequently have been the case, elevation was 
only another phase of the same undulation that had formerly given 
rise at the same spot to depression, and the directions of strain were 
approximately coincident with the direction of thrust. 
It is not meant to be suggested that the formation of joints has 
arisen through the action of a simple extensible force acting alone. 
Indeed, it is difficult to understand how any upheaval of the earth’s 
crust could well take place except under the impulse of an upward 
thrust of some kind or other. And if this thrust acted in the form 
of a series of undulatory impulses propagated from below obliquely 
outwards, their effect upon sheets of stony matter subjected to great 
lateral tension could hardly fail to result in the spacing-out of the 
rock by a series of joints. 
It may be objected to the view here set forth that a succession of 
earth-movements such as these must, in the end, inevitably result 
in joints of greater complexity in proportion to the number of times. 
the rock has been affected; and that, as I have myself stated that 
no certain traces of such effects can be discerned amongst our older 
and more highly-disturbed strata, the explanation here given cannot: 
well be the right one. The answer to this objection is that, in such 
eases, the subsequent tension is compensated by a further develop- 
ment of jointing, if one may so express it, in the form of faults, 
which have filled up the space by wedges of rock broken away from 
the parent mass along the pre-existing lines of weakness formed by 
the joints. ; 
- Joints in strata that have undergone metamorphism to such an 
extent that their older lines of weakness have been welded up, as 
well as the joints occurring in granite and rocks of similar mode of 
occurrence, present features different in some respects from ordinary 
vift-joints.. But as these special peculiarities have been already, as 
i think, satisfactorily accounted for by other writers, and there is 
nothing in their mode of occurrence that militates against the view 
here suggested, I am content to leave that part of the subject 
without further mention, and to confine my attention for the present: 
to the consideration of the special form of jointing whose origin it 
las generally been admitted is most difficult to account for. 
IY.—On tHe Bacsuor Sanps as A Source oF Warer SurPty. 
By the Rev. A. Irvine, B.A., B.Sc., F.G.S.; 
Senior Science Master in Wellington College. 
= ee the increase of population and the simultaneous pollution 
¥ of rivers in this country, the question of water-supply for the 
purposes of human life becomes every year a more serious one. 
