6 TIDESWELL DALE QUARRIES. 



In the quarry have been found slabs of a finely laminated 

 limestone, the upper sur- 

 faces of which are traversed 

 by a network of surface 

 cracks. Their appearance 

 is represented in Figure 

 4, which is a sketch of 

 a portion of a slab, 

 measuring about sixteen 

 inches in length, in my 

 possession. 



The question of the origin of these cracks is extremely 

 interesting. They resemble the cracks produced in the mud 

 bottom of a pond dried by the sun's heat. How can we 

 explain their production ? It has been suggested that they might 

 be produced by heated volcanic matter having been poured over 

 the mud. This is, I think, a very unlikely explanation. The 

 action of a stream of volcanic matter would be quite different. 

 Instead of being quietly deposited upon it and cracking its surface, 

 it would lead to considerable disturbance, and we might expect 

 that the junction between the two would be anything but sharp. 

 This seems the more certain when we reflect that, as the volcano 

 was submarine, the material over which the volcanic matter flowed 

 would be soft. Indeed, where we have been able to find the 

 junction between the toadstone and the limestone, as in Ember 

 Lane, near Bonsall, we find it to be characterised by a 

 heterogeneous rock, consisting of volcanic matter, enclosing 

 altered fragments of limestone. But the theory is completely and 

 for ever disposed of, by the discovery in the quarry, of slabs of 

 limestone, precisely similar in character to those containing the 

 cracks, but bearing upon their surface the casts of the cracks. It 

 could not therefore have been hot volcanic matter which produced 

 the cracks. What then ? The evidence is most striking and 

 conclusive. They are undoubtedly the cracks produced by 

 drying, and consequent shrinkage under the action of the 

 sun's heat. 



