Vol. 50.] ANNIVERSARY ADDRESS OP THE PRESIDENT. 12/ 



due, amongst other possible causes, to the original nature of the 

 containing rocks, of which, they say, it is difficult to speak with 

 certainty. An alternative but less probable supposition is, that it 

 may in part have been due to some peculiarity in the nature of the 

 intrusive rock. This is an acid granite with potash considerably in 

 excess of soda, very poor in ferro-magnesian silicates and abounding 

 in quartz. It has been subjected to considerable strain and 

 crushing, a circumstance which has suggested a comparison with 

 the rock of Bryn-y-Garn near St. David's, An analysis of this 

 granitite compared with that of the eurite of Cader Idris shows 

 that this is slightly the more acid rock of the two, whilst the pro- 

 portion of the alkalies is nearly reversed. 



Devonshire. — About half a dozen papers relate more or less to 

 petrological questions connected with this county, but as those 

 especially dealing with the volcanic history of the region have 

 already been noticed, it would be superfluous to allude to such 

 matters again. In his paper on the elvans and volcanic rocks of 

 Dartmoor, Mr. Worth has described two elvanite-dykes in the 

 neighbourhood of Tavistock for the purpose of demonstrating the 

 structural changes which they exhibit. One of these shows a 

 centre composed of quartz -felspar-porphyry passing laterally 

 through numerous varieties into ' claystone '-porphyry. The other 

 dyke, when traced for some distance, exhibits a change from a fine- 

 grained porphyritic granite to a rock with a compact semivitreous 

 ground mass, in which felspar, quartz, and mica are porphyritically 

 developed. 



Mr. Bernard Hobson has lately contributed a paper on the 

 Basalts and Andesites of Devonshire, known as ' felspathic traps.' 

 According to this writer it would seem that all but one of the 

 specimens examined are olivine-basalts. He feels no doubt of the 

 contemporaneous nature of the lavas exposed in all the localities 

 visited. This conclusion is in accordance with the views of 

 De la Beche, and contrary to the statement of Mr. Vicary that "they 

 commonly appear as dykes filling fissures in the earlier rocks." In 

 some places the basalt, as he calls it, may be seen to intervene 

 between the underlying Carboniferous and the overlying Permian (or 

 Triassic) rocks. Mr. Hobson doubts the connexion of these lavas in 

 any way with the Dartmoor granite, since the so-called ' felspathic 

 traps,' being really olivine-basalts, are not likely to be the effusive 

 equivalents of the granite. He also doubts the intrusion of quartz- 



