THE EVOLUTION OF DERBYSHIRE SCENERY. 20I 



saturated is spread out over a surface, as on the roofs of caves, or 

 in drops from projections, evaporation takes place, and the 

 carbonate of lime is deposited, hence the stalactitic and 

 stalagmitic deposits of limestone caves. In a similar manner we 

 may explain the formation of tufa, or of the " fur " deposited from 

 hard water in lea-kettles and hot-water pipes. 



This chemical action of water upon limestone, and all 

 rocks in which lime forms a constituent is important, 

 but it is obvious that its action will be modified by 

 the physical structure of the rock itself. No matter what 

 the solvent power of the water, it would be incapable of 

 producing the observed effects were it not for the numerous 

 joints and fissures which allow of the entrance of the 

 solvent. To the fact that these are numerous, and recur with, 

 some regularity in limestone, must be ascribed the character of 

 the scenery of many districts in Derbyshire. It is not sufficient, 

 however, to notice this structure on a large scale, but to explain 

 the peculiarities in the disintegration of various kinds of rock, 

 the minute structure must be observed. The capacity of rocks 

 for water varies very considerably owing to this difference in 

 structure. The water absorbed is in part taken in between the 

 constituent particles of the rock, and is retained until evaporated 

 or driven off by heat. This is termed the " water of saturation." 

 Other water passes more or less freely through the rock, and is 

 termed the " water of imbibition." Experiments have been made 

 by Prestwich, Wethered, Delesse and others, in order to 

 determine the quantity of water of saturation held by various 

 rocks. From these it is seen that while such rocks as slate and 

 limestone will absorb only a very small quantity of water, sands 

 and soft sandstones will absorb a large quantity, amounting in 

 some cases in sands to as much as three gallons per cubic foot. 

 Mr. Wethered, in the course of a series of observations, found 

 that millstone-grit from Sheffield absorbed over ten times as much 

 water as carboniferous limestone from Clifton. It is not difficult 

 to deduce the effects of freezing upon such rocks. During a 

 sharp frost the water contained between the particles of a sandstone 



