342 T. Mellard Reade — Pitted Pebbles in the Punter. 



show signs of distortion. This it never does in any of the examples 

 I have examined. A reference to the photograph will show this 

 clearly ; the depressions are perfectly sharp at the edges, and the 

 pebble retains its external shape. The material formerly. occuj)ying 

 tlie depression has been removed, not displaced. 



Second. — The pebble that is " indented " is often harder than the 

 pebble which indents. The "indenting" pebble is never distorted, 

 but is frequently fractured. 



Third. — Out of six indented pebbles now before me, separately 

 collected by myself, Mr. Henry Beasley, and Mr. Edmund Dickson, 

 when our observations were made at Cannock Chase in 1891, only 

 one shows signs of fracture — this is b shown in the photograph. It 

 is quite evident that the "fractures " are simply joints, such as may 

 be found in many pebbles, having no indentations upon them. This 

 pebble is a purple ferruginous sandstone or grit, almost as hard as 

 quartzite, but not having a lustrous fracture. The interior of the 

 " indents " is in most cases light grey, the purple colour having 

 been discharged — another evidence of solution. There are also four 

 quartzite pebbles from the Bunter of other localities sent me by 

 Mr. Beasley, one of which, measuring 2J" x If", has been fractured 

 and recemented by a deposit of silica which closes up about one 

 inch in length of the crack, and is consequently subsequent to it. 



Fourth. — Fractui'es are a sign that the material of the pebble is 

 rigid, and that it cannot be squeezed out of shape. Their existence 

 is to a certain extent evidence against the mechanical theory. 

 I think no geologist will contend that the pebbles have been 

 hardened and indurated since the}^ became pebbles ; their smooth 

 worn surfaces show that the rock they were formed from was iu 

 the same condition as the pebbles are now. 



Fifth. — The indenting pebbles perfectly fit the indents of the 

 pebbles. If the indents were the result of mechanical movement 

 this would not be likely to happen in all cases. The " indents " 

 in the ten pebbles before me vary from 1^ inches long by f inch 

 in width, shallow and pear-shaped, to circular pits only -rg- inch in 

 diameter. They are of all shapes, a saucer-like shape predominating. 

 Some of the pebbles are cemented into their places by a deposit 

 of silica. When the indenting pebbles are removed the cup or 

 depression is seen to be smooth, frequently having a deposit of 

 silica over the surface, sometimes one of carbonate of iron. This 

 deposit of silica in the case of the real quartzites is so hard that 

 a knife will not pierce it. 



Sixth. — The pittings are, in the more marked cases before me, 

 principally confined to one side and the edges of the pebbles. The 

 opposite side often has adherent somewhat loosely cemented sand 

 and small pebbles. I take this, which is on the flattest side, to be 

 the bed of the pebbles, and the pitted surface to be the top surface. 

 Why should this be the case on the mechanical assumption ? There 

 is not much evidence of lateral pressure in the stratum in which the 

 pebbles occur, and statical pressure one would expect to be equally 

 effective top and bottom. 



