Professor Bonney — Pebbles in the Trias. 77 



quartzites or grits at all common ; indeed, if I may reason from my 

 own experience, I should say there was about as good a chance of 

 finding them in the one place as in the other. 



Next as to the points of difference. On the whole, I think, 

 pebbles of about four inches diameter and from that to a foot are 

 rather commoner at Budleigh Salterton than on Cannock Chase. 

 Certainly in the pits on the latter one sees a smaller number of 

 the subrotund to subangular stones from 8 inches to a foot long. 

 There is also a rather marked difference in the form of the better- 

 rounded specimens. Pebbles which are nearly prolate spheroids 

 occur in both deposits, but that is the common shape in Staffordshire, 

 while those in Devonshire are more usually oblate. This is very 

 conspicuous in walking by the seaside along the pebbly beach, 

 probably because the specimens have been finished off, possibly even 

 to some extent selected, by recent wave-action. The eye is at once 

 arrested by a certain flatness in outline, which may be indicated by 

 saying that the dominant type of pebble is an ellipsoid, the diameters 

 of which are roughly as the numbers 5, 4, and 2. 



As to materials, vein-quartz pebbles are, I think, rather commoner 

 in Devonshire. Of the quartzites, both districts show several 

 varieties, varying from merely hard grits to rocks so uniform in 

 structure and so completely cemented as to break with an almost 

 smooth surface and subconchoidal fracture. The latter type, however, 

 is not common at Budleigh Salterton and is abundant on Cannock 

 Chase. Nevertheless, I found one or two specimens at the former 

 place which cannot be distinguished from the noted liver-coloured 

 quartzite of the latter. Slightly felspathic quartzites are commoner at 

 Budleigh Salterton, in fact a minute " speckling " of the stone is one 

 of the first things which catches the eye.^ The dark compact rocks 

 are rather common at Budleigh Salterton, not so on Cannock Chase. 

 In the former place they are very frequently almost subangular 

 and as large as the rest, pieces full 6 inches in diameter not being 

 rare : on the latter, they are generally rather small and better rounded. 

 Fragments chipped from five of the dark-green pebbles were powdered 

 and examined under the microscope.* All showed tourmaline, so 

 two of them were sliced. Both consisted mainly of granular quartz 

 and tourmaline ; the latter mineral occurring mostly in grains, some- 

 times almost films, occasionally in prisms and needles. The first are 

 generally brown in colour, now and then passing into a dull blue ; 

 their shape and aspect often suggest that the mineral originally was 

 a ferro-magnesian mica, and that the replacements may not always be 

 complete. The prisms vary from brown to blue, and the needles 

 appear to represent a paler variety than the othei's. Both these 

 forms are most abundant in small quartzose veins which traverse the 

 pebbles. Here sometimes the sides and one end of a crystal are 

 well developed. The general aspect of the specimens suggests 



^ Here also the lighter coloured stones become a whiter colour, after they have 

 been cleaned from the matrix, than they do in Staffordshire, though perhaps this 

 difference may not be of much value. 



* I am indebted to Miss C. A. Eaisin, B.Sc, for kind help in this matter. 



