78 Professor Bonney — Pebbles in the Trias. 



a sedimentary rock wliicli has been affected by contact-metamorphism 

 and has had its aluminous constituent converted into tourmaline. 



Another pebble, dark in colour, with thin bands or streaks of 

 a grayer tint, was sliced after tourmaline had been detected in the 

 powder. It consists mainly of quartz and the above-mentioned 

 mineral, but indubitable tourmaline is present in a quartzose vein. 

 In the paler bands quartz predominates ; in the darker mica or 

 tourmaline. In the latter are some small rounded spots of chalce- 

 donic quartz. These suggest the possibility of an organic origin 

 (? radiolaria), but I cannot venture to speak more positively. The 

 rock, I believe, was originally a stratified clay and silt, which sub- 

 sequently was affected by contact-metamorphism, followed by the 

 production of tourmaline. 



Lastly a pebble (from the beach) which exhibited well-marked 

 bands of a gray and a dull-brown colour, very minutely granular, 

 was sliced, after its powder had suggested the presence of tourmaline. 

 The lighter layers are mainly chalcedonic quartz with small films of 

 brown mica or tourmaline, the darker mostly of the last mineral, 

 which also occurs in grains and prisms, these being perhaps more 

 common than in the other specimens. Here also are veins with 

 prismatic and acicular tourmalines, but in the darker layers spots 

 or lacunge occur which are occupied by quartz and tourmaline, 

 sometimes prismatic, often acicular, the latter seemingly paler in 

 colour. The slice also contains some granules of a darkish mineral, 

 occasionally opaque, but sometimes rather translucent, and of a 

 muddy golden-brown colour, possibly cassiterite. This rock appears 

 to have had the same history as the last one. 



These specimens correspond with some of the fine-grained 

 tourmaline-bearing rocks which may be obtained from the zones of 

 contact-metamorphism around the granite masses of West Devon 

 and Cornwall. I did not, however, observe any of the coarse- 

 grained quartz-tourmaline rocks, which I have often seen in the 

 latter county ; for instance, near St. Austell and to the west of 

 Penzance. Dark green pebbles with a general resemblance to 

 certain of the above-named occur, as already stated, in the Midlands; 

 but these, so far as I have examined them microscopically, though 

 they contain tourmaline, present considerable structural differences, 

 and ai-e not suggestive of contact-metamorphism. 



Fragments of igneous rocks, while not abundant at either locality, 

 are distinctly commoner and more varied in Staffordshire ; there, 

 with the same amount of work, I could collect at least a dozen 

 varieties of quartz- felsite ; in Devonshire I found at most two 

 or three, only one of which was anything but rare. The same is 

 true of the more decomposed rocks, both basic and crystalline. In 

 Staffordshire I should have seen ten specimens of igneous rocks 

 where in Devonshire I saw one. In the former county also, what may 

 be called " miscellaneous sedimentaries," such as sandstones, mud- 

 stones, chert from the Carboniferous Limestone, etc., are commoner. 



I was not able to recognize any of the following rocks : the 

 gneissoid and hornblendic rocks, the serpentines or gabbros of the 



