MOYEMENTS OF KOCKS, ETC. 785 



caused by a partial sinking of the central regions, due to a reflex 

 action, the protracted adjustment of equilibrium after the great 

 mountain-features had been fully developed *. 



There is, I submit, another effect produced by the creeping move- 

 ment of large masses of rock, where, owing to the presence of preci- 

 pices or slopes, they are insufficiently supported on one or more sides, 

 and it is one which I believe to be of considerable importance. 



In the description of a creep given by Lyell reference is made to 

 the production of cracks in the pillars of coal left standing in mines. 

 These cracks are generally quite close, but very numerous ; they are 

 no doubt due to the strain induced by slight inequality in the 

 yielding of the bed supporting the coal, and thus have a precisely 

 similar origin to the joints and fractures artificially produced by 

 Daubree in different substances which he subjected to undulatory 

 movement by torsion, or to simple pressure f. 



Now the precipices of the Grand Canon district indicate in a very 

 remarkable manner the presence of joints and fissures. Over and 

 over again, in the descriptions given by the geological explorers of 

 these regions, we come upon expressions of surprise and admiration 

 at the extraordinary architectural forms into which the cliff faces are 

 carved J, and this is especially the case with regard to the higher 

 cliffs §. 



These sculpturings are, we know, mapped out by the joints and 

 fissures present in the sculptured masses [|, just as in a quarry the 

 readiest way of working the stone is determined by the positions of 

 the joints and fractures. If therefore we conclude with Daubree 

 that joints and fractures occurring in nature are due to small torsional 

 movements taking place in the rocks, we shall argue that some part 

 at least of the effects referred to have been initiated by joints and 

 fissures caused by creeping movements of the rocks due to their 

 position in the faces of precipices, i. e. to their want of support on 

 one side. 



An observation made by Mr. Dutton confirms this view, and seems 

 to indicate that fissuring produced by the small horizontal movements 

 of rocks thus situated has important consequences in facilitating 

 denudation. He tells us that he has repeatedly noticed that where 

 a fault runs in a direction perpendicular to the trend of a cliff, the 

 recession of the cliff is less on the side of the downthrow than on 

 the other side of the fault ^. 



It is manifest that the higher the cliff* the greater the superin- 

 cumbent weight upon the rocks at its foot, and the greater the 

 creeping movement and the jointing and fissuring consequent upon 

 this movement. This jointing and fissuring weakens the rock and 



* H. B. Medlicott,' Mem. Geol. Survey of ludia,' iii. pt. 2, p. 174 ; and Quart. 

 Journ. Geol, Soc. vol. xxiv. p. 48. 



t Geol. Experim. (Daubree), Part 1, section 2, chapter 2. 

 t ' Geology of the High Plateaus of Utah ' (Dutton), p. 254. 

 § ' Tertiary Hist, of the Grand Caiion District ' (C. E. Dutton), p. 204. 

 II ' Geol. Experim.' p. 324 ; and ' Tertiary Hist. Grand Canon District,' p. 53. 

 % ' Tertiary History of the Grand Caiion District,' p. 200. 



