552 Prof. T. 0. Bonney — North- Western Chamwood Forest. 



pyroclastic and on much, the same horizon as the Peldar Tor 

 porphyroid, for a well-grown plantation now occupies the ground. 

 Sharpley porphyroid crops out in the south-east part and is replaced, 

 in going westwai'd, hy a breccia, 1 the fragments in which represent 

 the ' purple porphyritic ' and the ' syenitoid ' mentioned in our 

 notes on the fragmental igneous rocks, 2 and their matrix, indeed the 

 rock as a whole, very closely resembles that on the summit of Bardon 

 Hill. Under the microscope the two are substantially identical. 

 According to my memory (confirmed by my notes) the rock further 

 west becomes more obviously pyroclastic, but until I can re-examine 

 that portion I will say no more than that a rock similar to the 

 ordinary one of Bardon Hill makes its appearance to the west as well 

 as to the south of the porphyroids. 



With Mr. Rastall, I once more visited the Swannymote rock. It 

 offers great difficulties, as our former description shows, 3 and some of 

 them have not been removed by the further study of specimens. The 

 occasional mottled aspect of the rock recalls the Bardon Hill breccia, 

 and its microscopic structure bears a close resemblance to that of the 

 Sharpley porphyroid, except that the phenocrystals of quartz and 

 felspar are rather smaller. But how are we to explain the presence 

 of the slate fragments ? To the unaided eye they do not appear more 

 altered than those common in other parts of the Forest, and under the 

 microscope they show no sign of contact metamorphism. It has been 

 suggested to me by Dr. Bennett and Dr. Stracey that a variety of the 

 Sharpley porphyroid might have broken through a bed of " slate 

 agglomerate ", which can be traced '/ from the north-east right round 

 the Forest " and " seems to mark a satisfactory horizon " ; 4 and has 

 swept these fragments along with it. This suggestion deserves very 

 careful consideration, for though the presence of these fragments and 

 their condition seem at first sight strongly in favour of a pyroclastic 

 origin, we may explain the absence of appreciable contact meta- 

 morphism by supposing the temperature of the intrusive magma to 

 have been comparatively low. 5 The basalts and dolerites of the 

 Fifeshire coast produce but slight changes in the rocks which they 

 have pierced and sometimes include as fragments. Limestones and 

 dark shales are hardened and the latter changed in colour, becoming 

 a greenish- grey ; sandstones are altered to a kind of quartzite, and if 

 red (as in Arran) are bleached, but that is all. They are never even 

 partially melted like the vitrified sandstones of Saxony. The Gross 

 Weilberg basalt in the Siebengebirge contains in its outer part 

 numerous fragments, apparently unchanged, of a trachytic tuff through 

 which it has broken. This explanation seems to me the more probable 

 because I cannot find any other evidence of a pyroclastic origin in the 

 Swannymote rock and am unable to separate its microscopic structure 

 from that of the other porphyroids in this north-western district. 



1 Well exposed in a little ridge running roughly north and south. 



2 Q.J.G.S., vol. xlvii, p. 95, 1891. 



3 Q.J.G.S., vol. xlvii, p. 85, 1891. 



4 Geology in the Field, p. 774. Possibly also, they think, the underlying 

 " felsitic agglomerate " may have been similarly treated. 



5 We noticed also that the slate fragments exhibited a rough parallelism. 



