All Unrecognised Agent in the Deformation of Bocks. 399 



the Bunter of Cannock Chase." Neither has heat had anything to 

 do with softening the rock, and the greatest pressure that could have 

 acted upon it is quite insufficient to have been the sole cause of the 

 crushing. We do not know how thick the Worcester Enon deposits 

 were ; they apparently are, and always have been very much thinner 

 than the rocks of the same age further east : to give the deposits a 

 thickness of a quarter of a mile, or 1,300 feet, would be "an outside 

 maximum. Mellard Eeade gives 15 cubic feet to the ton as the weight 

 of Bunter Conglomerate, t which is a rock similar to the Enon; a 

 quarter of a mile of this, then, would produce a pressure of -61 tons 

 or 1368*8 lbs. per square inch, while the crushing strength of a 

 compact rock like the sandstone would be at least equal to that of the 

 limestone, or more than ten times as much. In other words, to 

 crush the sandstone one would have to have the conglomerate 

 between two and three miles thick, which we can assuredly say 

 was not the case. 



The three cases of deformation having been found non-existent or 

 insufficient, how came the stone to be crushed ? To my mind the 

 marble slab that bends under its own weight affords an adequate 

 explanation. The stone is mostly made of quartz, and an ar-~ 

 gillaceous material consisting of minute scaly minerals, like kaolin 

 and chlorite, with perhaps a small amount of reformed felspar. A 

 certain number of these crystals will be in a position to give way 

 when any pressure comes on the stone, especially the small fiat 

 crystals of kaolin ; when these have slid as much as they are able, the 

 internal arrangement of the rock will be slightly different, and a new 

 set of grains will be in a position to give way when the next shock 

 comes along, and this process, repeated indefinitely in the course of 

 ages, would produce an accumulation of effects such as we see in the 

 Enon pebble. Apart from the cup of indentation, which may, or may 

 not, have been formed by solution, and which certainly has been 

 produced in certain cases by solution in limestone (Heim) and 

 quartzite (Gresley and M. Eeade), the cracks that radiate from the 

 indentation have been described before and have been figured in the 

 case of the last two rocks. In the quartzite, the cracks are clearer cut, 

 while in the limestone they are more irregular, and in my specimen 

 of sandy mudstone, they have granular broken edges. In both 

 limestone and quartzite the cracks are probably produced in the 

 same manner as I have described for the mudstone, namely, by the 

 constituent grains gradually giving way to stress. 



Finally, in the cleaved Dwyka pebbles we have deformation under 



* Geological Magazine, 1895, p. 239. 

 f Geological Magazine, 1895, p. 843. 



