HARDWICKE'S SCIENCE-GOSSIP. 



It should also be noted that in the hard beds the 

 cleavage-planes cross the beds at large angles ; the 

 line of least resistance of such beds would be 90° with 

 the plane of the beds, whereas, in the soft beds, the 

 cleavage forms acute angles with them. The line of 

 least resistance of such beds is parallel to the original 

 layers of the strata (compare also Science-Gossip, 

 J-SSi, fig. 144). From these facts we may deduce 



silica (S.G. = 2"6) in 1 ? Although the densities are 

 practically the same, there is a notable difference in 

 the general hardness of the beds, which at once 

 explains the refraction of the cleavage-planes. 

 Referred to Mohs' scale H=4*5 (i.e. between fluor 

 and apatite) in 1, and II = 2"5 (i.e. between rock- 

 salt and calcite) in a. 



It must be borne in mind that the refraction of 



Fig- 3-- 



'=• 3 1 - 



the following general law :— When cleavage-planes 

 pass from a hard to a less hard rock the planes are 

 bent away from the normal, or plane perpendicular, 

 to the surface of the beds, but when they pass from 

 a comparatively soft to a hard rock the planes are 

 bent towards the normal. The phenomenon is in 

 fact analogous to the physical law which rules the 

 refraction of a ray of light, but we are not here 

 dealing (as in the case with light) of media of vary- 

 ing densities, for I carefully ascertained the specific 

 gravities or densities of the beds 1 and a ; the former, 

 or hard bed, was 2715, and the latter 2763. Hence 

 the soft rock is actually rather more dense than the 

 hard one. Is this owing to the greater proportion of 



Fig. 3:. ' 



cleavage-planes is greatly complicated by the non- 

 homogeneity of rocks, and by the frequent pressure 

 of joints and vacant spaces in them. 



In the same neighbourhood some beds dipping 

 71 E., and striking N. 35 E. (magnetic) showed 

 cleavage-planes in soft beds 83 E., and in thick hard 

 beds 64 W., or a refraction of 33 . The lines in the 



