OF PHYSICAL GEOGRAPHY. 13 



stance of rocks to its exit in a spring, without teaching us that 

 these rocks are continually undergoing waste, and that this waste 

 is proportional to the nature of the rocks. Rain-drops bring 

 down carbonic acid, and thus exert a chemical as well as mecha- 

 nical action. In favourable circumstances, the actual channels 

 which they make are preserved. On the wide and bare surfaces 

 around Ingleborough and Penyghent, and on Hutton Roof Crags, 

 west of Kirkby Lonsdale, these channels are innumerable, of all 

 breadths and depths, and of lengths and direction depending on 

 the -slope and continuity of the masses. Where the strata are 

 level, the little ramifications of the rain-channels run deviously, 

 and terminate in the numerous natural joints ; but where, as on 

 Hutton Roof Crags, the strata acquire a steep arched slope, the 

 channels take the direction of the slope, run together as valleys 

 do, and collect into miniature dales, till some great fissure lying 

 across their path swallows them up. Below this joint, other 

 channels commence to be in their turn swallowed up (see Geol. 

 Proc. 1831, vol. i. p. 323). 



The fissures here indicated are natural joints of the rock, pro- 

 duced by contraction during its consolidation ; they are often 

 symmetrically disposed (prevalent directions are N.N.W. and 

 E.N.E.), and by dividing the mass of the limestone present easy 

 passages downward for water. Thus Malham Tarn delivers itself, 

 not by a surface-channel, but by subterranean passages: the 

 river Nid is swallowed up near Lofthouse : streams which gather 

 on the moorland fells, sink into smaller holes of the limestone 

 below, or wind through subterranean caverns. These fissures, 

 by giving passage to water, suffer enlargement so as to become 

 rifts between cliffs, or channels round insulated peaks or jutting 

 crags. Gordale, a good example of these effects, will again 

 attract our attention. (See the Lithograph.) 



When the fissures have one prevalent direction, the rock is 

 split into vertical plates : a second set of joints developes prisms 

 in these. Large joints, thus crossing at intervals, produce huge 

 vertical masses, which, in consequence of the removal of adjoin- 

 ing parts, often stand out like prominent towers of a Cyclopean 



