160 



But for our present object the rock of most importance as 

 regards its behaviour before subaerial agents is Hmestone. Not 

 much more need be added to what has been already stated about 

 the cutting back of a waterfall in this rock : where, however, lime- 

 stone is found in thick beds, and is not very much split up by 

 structural planes, it seems to recede not quite as fast as sandstone 

 does under like conditions. But under the influence of the 

 weather, limestone, as is well known, often wastes with great 

 rapidity. Jukes's comparison of it to a glacier melting before the 

 summer's sun conveys an excellent idea of the way this rock is 

 dissolved and carried away in solution by the waters from the 

 surface. The numerous structural planes that intersect every bed 

 of limestone are developed, and rapidly widened, to a considerable 

 depth from the surface, by the corrosive action of percolating 

 waters, which thus easily find their way downwards and are enabled 

 to attack rock at a lower level. 



There seems reason for believing that the absolute rate of 

 dissolution of limestone is far from slow, even when measured by 

 years. In Kirkby Stephen Churchyard in 187 1, there was an 

 erect gravestone of ordinary Mountain Limestone that had then 

 been put up about fifty years. As the stone was carved, at least 

 the greater part of it must once have been smooth and unweathered; 

 but when I saw it in 1871 there were encrinite stems, and bits of 

 other fossils, left in relief to the extent of a tenth of an inch or 

 more, the softer matrix having been removed by the rain that had 

 fallen on the stone since its erection. One cannot be quite sure 

 even that the highest parts of the fossils accurately represented the 

 original dressed surface ; but, assuming that they did so, we have 

 in this instance proof that a smooth and quite-unwealhered piece 

 of limestone, standing in a position the least favourable for erosion 

 by subaerial agencies, had been dissolved away at the rate of one 

 inch in five hundred years.]* 



The foregoing statements will, I think, suffice to shew that the 

 relative degrees of destructibility of limestones, sandstones, and 



* The paragraphs included between the square brackets are reprinted, without 

 essential alteration, from the Gnl. Mag. 1875, pp. 325-6. 



