HARDWICKE'S SCIENCE-GOSSIP. 



201 



sing them, as a general rule correspond very nearly. 

 Examine plate ix. of the late Mr. Ward's excellent 

 memoir.* It shows the direction of the anticlinals, 

 and synclinals in the true dip, and cleavage dip — 

 they are parallel, or form very acute angles with 

 each other. The cleavage of the Skiddaw slates, 

 (Lower Silurian), which are often much contorted 

 and crumpled, is constant in direction (N.E.), in dip 

 (S.E.), and amount of dip (between 55 and 6o°). 

 In the volcanic series the cleavage dips as high as 

 70°. There are exceptions to the above rules. 

 Many years ago, De la Beche gave one example 

 occurring at Sharkham Point, near Brixham, Devon. 

 Here the direction of cleavage makes somewhat large 

 angles with the planes of bedding, f When cleavage 



strata. Action and reaction being equal and opposite, 

 our observations are evidences just as much of varia- 

 tion in the direction of resistance to pressure. The 

 major plication of strata, are usually termed anti- 

 clinal and synclinal curves, but just as each mighty 



Fig. 122. 



planes run parallel with planes of bedding, no refrac- 

 tion or bending of the planes is likely to take place ; 

 when the angles formed by the two planes are very 

 small, the refraction, if any, will be very slight, so 

 slight as frequently to be overlooked, but when the 

 angles are larger, no doubt the cleavage-planes are 

 bent when passing obliquely from a hard to a soft 

 bed, or vice versa. 



We have now to consider what appear to be 

 evidences of variation in the direction of the pressure- 

 forces which have contorted and cleaved certain 



* Geology of the Northern Part of the English Lake 

 District, 1876. 

 + Report on the Geology of Cornwall and Devon, fig. 3, 



P- 45- 



swell of the ocean is made up of a number of lesser 

 waves, so each great rock-curve is usually composed 

 of a number of smaller folds. The latter are termed 

 troughs and saddles — words which explain themselves. 

 The smaller folds of rocks may be distinct and 

 regular, or greatly broken and confused. Fig. 120 is 

 a rough diagram of a saddle existing on the top of 

 the hill marked Pen-y-Bannau on the geological 

 survey map (quarter-sheet 57. N.E.). A little north 

 of the Roman camp the beds are marked contorted. 

 The saddle occurs about one hundred yards to the south 

 of the camp. There is no distinct axis or saddle- 

 joint, but here and there are very palpable evidences 

 of fracture, and the dotted line shows that the 

 ruptures have not taken place in the same plane. The 



