578 SCIENCE PROGRESS 



holding the thumbs below and the four fingers above ; the reverse 

 fold on the interior can easily be produced, but the drag on the 

 outer layers can be felt. 



The second illustration I take from the Kouga Mountains, 

 which lie to the east of the Zwartberg range. These form a wild 

 and inaccessible block of country, traversed by wild kloofs and 

 precipices, and were the last stronghold of the Bushmen in the 

 south-west of the Colony. A narrow slip of Devonian shales lies 

 folded in with the sandstone on the southern flanks, and the 

 abrupt syncline shows many of the typically strange folds with 

 which we are dealing. The particular fold I illustrate lies along 

 the Schrikke river, and shows the topmost bed of the sand- 

 stone folded into a knee-bend, the southern limb dipping south 

 at about 30° and the northern nearly vertical. The beds below 

 the resistant bank have been caught in the nip and have been 

 brecciated ; only a small block of stratified sandstone appears 

 through the rubble, representing, apparently, part of the rocks 

 which were originally near the actual axis, and which have 

 reached their present position by having been crushed back- 

 wards. 



There are three ways of regarding this flexure : as having been 

 produced by (1) the rock having been molten, (2) the rock 

 having given way along slip or gliding planes, (3) the rock 

 grains having been transposed by solution and redeposition. 

 The first is clearly disproved by the conditions of the rocks ; 

 the second is an agent in the deformation of the rocks which is 

 a potent factor in mass static conditions, but not, I think, under 

 the conditions that prevail in rock-bending ; while the third, 

 which I assume here is the principal agency of rock deformation, 

 renders the term " tensile strength" somewhat misleading. The 

 actual tensile strength of rocks is measured by the modulus of 

 elasticity, which in sandstones works out at anything between 

 30,000 and 400,000 lb. per square inch ; that is, a force repre- 

 sented by these figures would be required to stretch a bar 

 of the rock a square inch in section to twice its length. This is 

 an actual property of the rock as we quarry it, but the tensile 

 strength, used in the sense in which I have done, refers to the 

 resistance to solution and redeposition, and can only be used 

 comparatively, with regard to other rocks which simultaneously 

 were subjected to the same temperature and pressure for the 

 same time. It is, perhaps, more a co-efficient of the porosity, or 



