19-1 



MR. H. H. THOMAS ON THE 



[May 191 i r 



present undoubted trachytic affinities despite the excess of soda 

 over potash. Moreover, these more acid rocks are readily dis- 

 tinguished in the field from more basic lavas, which must be re- 

 ferred to as keratophyres. In this paper, therefore, the term 

 keratophyre is restricted to those soda-rich lavas which would 

 fall between the soda-rhyolites and the spilites, and differ chemically 

 as well as structurally from the rocks hitherto described. 



They are strongly represented on Skomer Island and the main- 

 land, and are identical in most respects with the ' lime-bostonites ' 

 of Dr. Elsden, 1 which Prof. Rosenbusch " places with the kerato- 

 phyres. In the case of Skomer the name ' lime-bostonite ' seems 



Fiff. 8. 



A = Keratophyre with albite phenocryst and chlorite-filled vesicle. Above the 

 soda-trachyte of North Castle (Skomer). Slide E 7024. X 25 diameters. 



33 = Keratophyre, non-porphyritic and non-vesicnlar. Below the soda-rhyolite 

 of Bull Hole (Skomer;. * Slide E 7128. X 25 diameters. 



unsuitable, for the rocks are certainly not intrusive like the similar 

 rocks in North Pembrokeshire ; nor is their structure that which is 

 met with in typical bostonites. 



As represented on Skomer and the adjoining mainland, they are 

 fairly compact, grey to buff-coloured, somewhat speckled rocks, with 



1 Q. J. G. S. vol. lxi (1905) p. 594. I am indebted to Dr. Elsden for kindly 

 placing at my disposal his type-sections of rocks from the Abercastle district 

 of North Pembrokeshire. 



2 'Mikroskop. Physiogr. d. Miueralien & Gesteine' 4th ed. vol. ii, pt. 2 (1908) 

 p. 944. See also his ' Elemente der Gesteinslehre ' 3rd ed. (1910) pp. 345-46. 



