208 



H. Dewey Sf Dr. J. 8. Flett- 



( formation of greisen, etc.) while others are practically free from tliem. 

 Bailey & Grabham (24) showed that the quartz-diabases of Scotland 

 are occasionally albitized, but only locally and incompletely. 



Picrites. 

 Several picrites have been recorded from Cornwall and Devon, 

 mostly hornblende picrites. Similar rocks occur in the Devonian of 

 the Rhine. They contain a little felspar, usually labradorite or 

 bytownite, and, as in the quartz-diabases, it is not albitized. 



ANALYSES OF ALBITE-DIABASES, MINVEEITE, AND PICBITE. 



100-13 99-97 100-73 100-70 



I. Gwavas Quarry, Newlyn, Cornwall (analysed by W. Pollard). " The 



Geology of the Land's End District " : Mem. Geol. Surv., 1907, p. 35. 

 II. Albite-diabase, Trusham Station Quarry, west side of Teign, Devon (analysed 



by E. G. Eadley, E. 6808). 

 III. Proterobase, 200 yards north of viaduct, south-west of Cartuther, Cornwall 

 (analysed by W. Pollard). " The Geology of the Country around 

 Plymouth and Liskeard " : Mem. Geol. Surv., 1907, p. 100. 

 IV. Hornblende picrite, Crime Mill, Molenick, Cornwall (analysed by E. G. 

 Eadley). Ibid., p. 101. 



The rocks that we have considered are of basic or ultra-basic 

 composition, but acid and intermediate rocks also accompany the 

 pillow-lavas. Of these we may distinguish the keratopbyres (and 

 albite-trachytes), quartz-keratophyres (and soda-felsites), and the 

 soda- granites. 



Keratophykes, etc. 



The keratopbyres are typically quartz-free. They consist of albite, 

 chlorite, and iron-ores, and the structure varies from orthophyric to 

 porphyritic; the phenocrysts are anortboclase or albite. First 

 described by Teall from the Southern Uplands of Scotland (25), and 

 subsequently from Mevagissey in Cornwall (26), they have been found 

 also in association with the pillow-lavas of Tayvallich (27), and the 

 variolites of Fishguard (28). Quartz-keratophyres contain usually 

 phenocrysts of corroded quartz and albite or anortboclase, in a matrix 

 that may be felsitic or microcrystalline. The soda-felsites are often 

 strikingly perlitic. Quartz-keratophyres are common in Devon with 



