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Indiana University Studies 



Out of six pebbles examined only one showed signs of 

 fracture. 



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

 may be found in many pebbles. . . . Fractures 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. 



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. . . . When the indenting pebbles are removed 

 the cup, or depression, is seen to be smooth, frequently having a deposit 

 of silica over the surface (the pebbles were quartzite), sometimes one of 

 iron. 



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? 



This writer showed that the maximum amount of static 

 pressure at any time on the Bunter rocks would not have 

 been sufficient to crush or disturb the pebbles of the con- 

 glomerate. There is practically no evidence of lateral pres- 

 sure in the locality. 



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 depres- 

 sions there is a deposit of silica which smooths the surface of the depres- 

 sion and unites the grains of the rock. In some places where a joint or 

 crack traverses the depression, the silica fills it up. The grains of silica, 

 where they are seen in the depressions of true quartzite pebbles, show 

 like a mosaic, and appear to be flattened or cut olf on their upper sur- 

 face. I think it extremely probable that solution and deposit have gone 

 on alternately. The solution of the silica has taken place, there is evi- 

 dence on all hands, including the adherent sand and gravel, for solution 

 Inust 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. 



Gresley (1895, p. 239), in a letter to the Geological Maga- 

 zine, described an indented pebble of the Bunter conglomerate, 

 which he claimed bore unmistakable evidence that the im- 

 pressions were produced by squeeze and pressure, inasmuch 

 as the pebble was not only severely fractured into four or five 

 pieces, but was also minutely faulted. The lines of fracture 

 radiated from typical indentations upon opposite sides of the 

 pebble. 



