344 T. Mellard Reade—PitUd Pehhles in the Bunter. 



The following is an analysis of one of the Cannock Chase pebbles 

 given in my original paper, and made by Mr. P. Holland, F.C.S., 

 a member of the Liverpool Geological Society : — 



Analysis of an indented pebble from Stile Cop Gravel Pit, near Eugeley. 



SiOa 



AlsOa+TiO. 



FeoOs ... .: 



MnO 



CaO 



MgO 



K2O 



NaaO 



Combined -water 



100-14 

 As further illustrating solvent action at the points of contact, 

 I may refer to a pebble of hard compact grey limestone (probably 

 Carboniferous), 3 inches in diameter, 1^ inches thick, rounded and 

 disc-like in form, from the Bunter Conglomerate near Wolsley 

 Bridge. Thei'e are two well-marked dish-like depressions in the 

 stone respectively 1 inch and f inch in diameter, which are coated 

 with a thin deposit of carbonate of lime. There is also a good 

 deal of adherent quartzose sand cemented together with carbonate of 

 lime. Here the pebble is of limestone, not quartzite or sandstone; 

 yet the same kind of action has gone on, namely, solution of the 

 limestone at points of contact and re-deposit of the carbonate of 

 lime. There is not the slightest sign of fracture to be seen. 



EXPLANATION OF PLATE XI. 



Fig. 1. — Photograph of three boulders or pebbles from the Bimter Conglomerate 

 of Cannock Chase. 



a. Very hard, fine close-grained grit, boulder, or pebble, 7|in. x 6in. X 4 in., 

 well rounded and -worn. It is covered with ' ' pittings ' ' ; the prominent 

 circular saucer-like depression which shows in this, and more plainly in 

 Fig. 2, is § inch across. There is a shallow depression IJ inches long on 

 the upper part of the pebble which is not seen. These pittings are found 

 as small as xt inch in diameter, but embedded grains of quartz have been 

 found only J;; inch in diameter. The pebbles, Avhere entirely clear of 

 adherent sand, are quite sharp at the edges, as may be seen on examining 

 the plate with a lens. The surface of the pittings is mostly granular, but in 

 some cases has a deposit of silica upon it. The projections above the general' 

 surface of the boulders are either whole pebbles, broken or decayed pebbles, 

 or groups of pebbles with sand between cemented thereto. The cemented 

 sand sometimes makes a rim round the depression. Most of these small 

 pebbles, with which the large one is roughened, are counter-sunk or embedded 

 in the boulder, and if removed would disclose pittings. They are generally 

 very firmly cemented into the depressions with silica, and fit them exactly. 

 The opposite side of the boulder has a good deal of adherent sand and 

 gravel, but not many "pittings," which are pi'incipally confined to the 

 surface shown in the photograph and to the edges. (Stile Cop.) 



h. Hard, fine-grained, purple-brown coloured grit boulder, or pebble, full of joint 

 planes, along three of which it has split ; these broken joint surfaces are 

 covered with ferric oxide. The "pittings" are very numerous, and are of 

 a lighter colour than the surface of the pebble, which is well water-worn. 

 One of the pittings is canoe-shaped, 1^ inches long by ^ inch wide. There 

 are small pebbles adherent and imbedded ; in this and most other respects 

 the description of boulder a applies to boulder b. (Stile Cop.) 



