■ Rev. A. Irving — Water Supply from the Bagshot Beds, etc. 17 



the cleavage-planes over an extended region, but that was w^ith the 

 supposition, since abandoned, that the cleavage-planes were the 

 actual surfaces of shearing : the amended theory gives much less 

 satisfactory results. Thus, let be the angle between the cleavage 

 and shearing planes at any given place ; then it is easy to prove that 



tan = V— . 

 a 



This shows that where the shearing and consequent distortion are 

 slight, the angle between the directions of shearing and cleavage 

 will be nearly 45°, and although it is less for a greater amount of 

 shearing, still, even when the shear is so great as to produce a dis- 

 tortion in which a = 6c, the angle is as much as 22°. 12 . If we 

 imagine a tract of rocks upheaved to a considerable height and 

 partially settling down with a shearing motion, and if we believe 

 with Mr. Fisher that such shearing is the cause of cleavage in the 

 rocks, it appears from the foregoing formulfe that the result should 

 be similar to that depicted diagrammatically in the figure. At th6 

 points A, C, E, there is no shearing and therefore no cleavage ; in 

 the neighbourhood of these points there are broad bands of rock 

 with no sensible cleavage-structure. Passing beyond, these bands, 

 the planes of ill-developed cleavage have a hade of about 45° : the 

 cleavage becomes gradually more marked and its hade slightly 

 steeper towards the points B, D, where the shearing is greatest ; and 

 near these points the distortion of fossils is greatest and the angle 

 the cleavage-planes make with the vertical is least. 



Now I submit that all this is very different from anything recorded 

 as occurring in nature. 1 have supposed the planes of shearing 

 vertical ; if they are inclined outwards as in the figure on p. 275, 

 the discrepancy between theory and observation is still more marked, 

 for the hade of the cleavage-planes from the vertical is then greater. 



IV. — Water Supply from the Bagshot and other Strata. (No. 2.) 



By the Eev. A. Irving, B.Sc, B.A., F.G.S., 



of Wellington College. 



DURING the year 1883 I gave some account, in the pages of this 

 Magazine,^ of the peculiarities of the water drawn from the 

 Bagshot strata of the London Basin. The most important con- 

 clusion drawn in that paper from the evidence adduced was the great 

 difference in the quality of the water according as it is obtained 

 either (a) from the bright ferruginous sands of the Upper Bagshots, 

 or (b) from the Middle or Lower Bagshot strata, which are all 

 contaminated (more or less) by vegetable matter in various stages 

 of decomposition. So far as the evidence in my possession a j^ear 

 ago seemed to carry me, I felt warranted in making the following 

 generalization : " The whole of the well-water of the district 

 which common human experience pronounces wholesome is ob- 

 tained from the Upper ferruginous sands." This has been confirmed 

 by additional facts more recently learnt. 



1 Vide Geol. Mag. 1883, Dec. II. Vol. X. pp. 404-413. 



DECADE III. — VOL. II — KO. I. 2 



