I 



THE ORIGIN OF DERBYSHIRE SCENERY. 185 



well show the effects of minor bedding planes. The planes 

 of stratification must be referred to interruptions in the depo- 

 sition of the material which formed the rock, to changes in 

 the nature of this material, and other causes. Joints are 

 crack-like divisional planes, generally crossing the planes of 

 stratification at a high angle. These are probably partly due 

 to slirinkage caused by the drying of the rocks, and partly to 

 internal stresses and movements. They are present in igneous 

 as well as sedimentary rocks, and the writer dealt with their 

 production in the former, in the Journal for 1887. It is by 

 means of these joints that rocks may be removed in blocks in 

 quarrying, and to them we owe some of the most familiar features 

 of Derbyshire scenery. They generally run in two directions at 

 right angles to each other, and so make it possible, by taking 

 advantage of the planes of stratification to remove the rock in 

 large quadrangular blocks — indeed, the art of quarrying largely 

 consists in taking advantage of these planes of weak cohesion. 

 Sometimes the joints are open, but most frequently they are 

 invisible. They permit entrance, however, to rain water, and 

 this, in virtue of its chemical and mechanical properties soon 

 widens the joint, and renders evident its previous existence. In 

 virtue of the carbonic acid which rain water dissolves from the 

 air and decaying vegetable matter, it possesses the power of 

 dissolving the hmestone, while, in freezing, its expansive force acts 

 powerfully as a wedge in breaking up the mass. In Mr. Ward's 

 article on Rain's Cave in l\\e Joiirual (or 1889, it gives an interest- 

 ing example of the underground evasion of limestone. To these 

 and certain other agencies must be attributed the gradual erosion 

 of the surface, the formation of many of our valleys, and also 

 many local phenomena. Take the case of the landslips which 

 have occurred at Crich. Crich hill, which is somewhat remark- 

 able in form, being of the shape of an elUptical dome, consists of 

 mountain Hmestone. The western and south-western sides are 

 steep, and below them lie the softer Yoredale shales. The 

 limestone is faulted against the Yoredale strata, and these 

 softer rocks have been worn down and carried away by 



