Corresjjondence — Mr. W. 8. Grcdey. 239 



THE INDENTATION OF THE BUNTER PEBBLES. 



Sir, — In "Annals of British Geology," 1892, p. 52, is a review 

 01" abstract of a paper entitled "The Trias of Cannock Chase," by 

 T. M. Eeade. In this abstract we read, "The indentation of the 

 pebbles he considers to be the result of contact-solution, the water 

 being retained at these spots by capillary action." 



Now, whether this theory of the origin of the pittings or little 

 surface dimples on some of these pebbles is a probable one or not 

 I do not discuss, but a specimen of one of the Cannock Chase 

 pebbles I happen to possess (picked up by myself in the ballast on 

 the London and North-Western Railway, near Lichfield, about eleven 

 years ago) bears unmistakable evidence that the indentations upon 

 it were produced by a squeeze or pressure ; for this reason one end 

 of the pebble (a reddish-brown quartzite, by the way) is not only 

 severely fractured into four or five pieces, but is also miniature 

 faulted. The lines of fracture radiate from typical indentations or 

 dimples upon opposite sides of the pebble. Here, then, at all events, 

 is an actual instance of a Bunter pebble not only indented by other 

 pebbles in contact with it in situ in the conglomerate, but it is 

 evident the pressure upon its sides was great enough to split it 

 and dislocate its fragments. The once-open cracks, so produced, are 

 now filled with calcite, which acts as a cement, holding the several 

 portions of the pebble together. This fact supports the physical or 

 mechanical origin of the indentations of the Bunter pebbles. Can 

 Mr. Reade adduce evidence in support of the chemical theory, of 

 which it does not appear he gave any in his paper? 



Here is a sketch of one side of the particular pebble I refer to. 



X = One of the indentations -^g in. long. 



X ' X " Lesser indentations -witli finer cracks in contact therewith. 



c, c, c, cracks in pebble, now filled with calcite. 



Note. — Corresponding indentations on opposite side in a larger 

 one than this. 



Erie, Pa., U.S.A., 1th Februanj, 1895. W. S. Gkeslky. 



