T. Mellard Reade — Pitted Pebbles in the Bimter. 343 



Seventh. — Let us try what the maximum statical pressure at any 

 time on these Bunter rocks may have been. If we assume that 

 a mile in thickness of rock at one time existed over the Bunter 

 Conglomerate of Cannock Chase. I think it will not be above the 

 mark. At 15 cubic feet to the ton, a column of rock a mile high 

 would give 352 tons to the square foot. The crushing weight of 

 granite cubes is about 720 tons per square foot on the average. 

 Quartzite is harder.^ If the whole pressure of the column of rock 

 were equably distributed, it would necessarily neither crush nor 

 disturb the quartzite. If, on the other hand, the pressure was 

 increased by being concentrated on certain points, fracture, not 

 distortion, would be the result, from the other parts of the pebble 

 being unsupported. The smaller the indenting pebble the more 

 cogent is this argument. 



It is quite remarkable how numerous minute pebbles leave their 

 marks on the very hai'd sandstones. Pressure applied to them in 

 such a way as to tend to stamp them into the boulder would 

 inevitably crush them, for it is a condition of indentation by 

 pressure that the pressure cannot be applied in a manner to prevent 

 crushing. There seems to be little relation between the hardness 

 of the pebbles and the existence of the indentation. One small 

 pebble, a specially light sandstone showing casts of fossils, has two 

 Well-formed pittings in it, and evinces no sign either of fracture or 

 crushing. 



Eighth. — But we have positive evidence that at the points of 

 contact of the pebbles solution and deposit have been going on. In 

 most of the depressions there is a deposit of silica which smooths 

 the surface of the depression and unites the grains of rock. In 

 some cases, when a joint or crack traverses the depression, this 

 silica fills it up. The grains of silica, where they are seen in the 

 depression of true quartzite pebbles, show like a mosaic, and 

 appear to be flattened or cut off on their upper surface. I think 

 it extremely probable that solution and deposit have gone on 

 alternately. That solution of the silica has taken place, there is 

 evidence on all hands, including the adherent sand and gravel, for 

 solution must precede deposit. Solvent action would concentrate 

 itself on the continually damp spots, and these are the points of 

 contact of the pebbles, especially on the upper surface of the larger 

 pebbles. The Bunter is a water-bearing rock, and water has a free 

 circulation through it. Eeferring to the figure given by Mr. Gresley, 

 I would respectfully submit that the branch fractures from the 

 depression are not a proof that the depression was due to the same 

 cause as the fracture. One pebble could not be driven into another 

 without distortion of one or both, as already explained ; and the 

 fractures are a proof of the pebble giving way and breaking up 

 without distortion. Also the fractures are not shown to cross the 

 depression or indent. In none of my examples are there any 

 radiating fractures. 



' Quartz -rock, Holyhead, is s^iven by Mallet as 1641 '6 tons per square foot across 

 laminations, and 900 tons parallel to laminations. — Manual of Eocks, Tables, etc., 

 D. K. Clark, p. 631. 



