630 



GEOLOGY. 



for the atmospheric and aqueous agencies, which have subsequently operated, so far from 



giving them a smoother outline, wear away the 

 surface unequally, producing furrows and pro- 

 jections. It often appears, that the pressure 

 from below, acting with inferior intensity, 

 has lifted up strata without any consequent 

 fracture, and has put forth greater power at 

 two neighbouring points than in the inter- 

 mediate space. This has given rise to valleys 

 of Undulation, of which the accompanying sec- 

 tion is an example from the Jura mountains. 

 In what are termed valleys of elevation, there 

 has been fracture, with an upward movement 

 of the fractured part, the strata dipping from 

 the valley on each side ; to the formation 

 contributed. Dr. Buckland noticed a valley 



of 



Section of the Jura Mountains. 



which, denudation has subsequently 



of this kind, of which fig. 29. is a section : v, valley of Kingsclere, near Shaftesbury; 



a a, chalk, with and without flints; c c, green-sand, strata which evidently were once 

 continuous. There is little difference between valleys of elevation and dislocation ; but 

 the former term is generally applied to those which are bounded by hills of moderate 

 height. 



Both the stratified and unstratified rocks exhibit a peculiarity of structure, to which 

 the term joints is applied : but this is more especially true of the sedimentary deposits, 

 each stratum being divisible into masses of determinate shapes, generally approaching to 

 the cubical or rhomboidal form ; and by these separations, or tendency to separate, the 

 quarrying of a rock is materially facilitated. Some rocks also are divided by a set of 

 parallel planes, coincident neither with the stratification, the lamination, nor the joints, 

 to which the name of cleavage has been given. These cleavage planes are most perfectly 

 exhibited by the argillaceous slates, and in those of the finest grain. They are remark- 

 able for maintaining an almost exact parallelism while ranging over many square miles 

 of country, and for preserving this parallelism across wavy and contorted strata. In 

 fig. 32. the planes of stratification, a a, are exhibited ; the joints, b b, and the slaty cleavage, 

 cc\ but the cleavage structure is more distinctly displayed v&fig. 31., from the slate rocks 

 of Wales. Professor Sedgewick, who has paid the most attention to this subject, remarks, 

 in elucidation of it : "A rugged country, more than thirty miles in length, and eight or 

 ten miles in breadth, stretching from the gorge of the Wye, above Rhaiada in North 

 Wales, to the upper gorges of the Elan, exhibits, on a magnificent scale, thousands of 

 examples like that represented above. The whole region is made up of contorted strata, 

 and of the true bedding there is not the shadow of a doubt. Many parts are of a coarse 

 mechanical structure ; but subordinate to them are fine crystalline chloritic slates. But 



