British Pilloic-lavas. 



209 



the pillow-lavas there, and soda-felsites occur in the Southern 

 Uplands of Scotland. 



Soda-granites. 



The soda-granites are coarse-grained rocks, generally with pheno- 

 crysts of albite. The matrix is a holocrystalline mixture of quartz 

 and albite. Ferromagnesian minerals are rare, hut include biotite and 

 pyroxene, the latter represented only by pseudomorphs. Rocks of 

 this class occur at Porthallow in South Cornwall, where they are 

 intrusive in the Ordovician pillow-lavas ; and at Tay vallich in 

 Argyllshire they penetrate the spilites that are interbedded with the 

 limestones and schists of the Central Highland metamorphic series. 



ANALYSES OF KERATOPHYBES, SODA-FELSITES, Etc. 



I. 



II. 



Ill 



IV. 



V. 



100-27 100-45 100-27 100-17 100-21 



Keratophyre, Hamilton Hill (analysed by J. J. H. Teall). " The Silurian 



Bocks of Scotland " : Mem. Geol. Surv., 1899, pp. 88-9. 

 Keratophyre, Trevennen (analysed by W. Pollard). " The Geology of the 



Country around Mevagissey " : Mem. Geol. Surv., 1907, p. 56. 

 Soda-rhyolite, west side of Skorner Island, top of cliff west of the spit 



(analysed by E. G. Badley, E. 7768). 

 Soda-granite, Tayvallich, Argyllshire (analysed by E. G. Badley, S. 13221). 

 Soda-granite, north corner of Porthallow Cove, Cornwall (analysed by 

 E. G. Badley, E. 6377). 

 We have, then, a long and varied suite of igneous rocks belonging to 

 the spilite association. The commonest are always spilite and diabase; 

 next in abundance are keratophyres and quartz-keratophyres; less 

 frequently we find soda-granite, minverite, and picrite. They range 

 from ultra-basic rocks like picrite to highly acid quartz-keratophyres, 

 and their chief characteristic is the abundance of soda, even in the 

 basic types. A close parallel can be drawn between them and the 

 picrites (teschenites), dolerites and basalts, trachytes, and rhyolites of 

 the Carboniferous rocks of Scotland (29). In both series felspar-free 

 rocks occur (picrites and peridotites) and quartz-diabases, otherwise 

 the spilite suite is always the richer in soda, and shows an additional 

 characteristic, the almost universal and often complete albitization of 

 the lime-soda felspars. 



(To be concluded in our next Number.) 

 DECADE V. — VOL. VIII. — NO. V. 14 



