266 Proceedings of the Royal Physical Society. 



lath-shaped plagioclase. The felspar phenocrysts are basic 

 labradorite. The rock is very porous, and the nature of the 

 ground-mass is rather obscured by products of decomposition. 

 It consists mainly of laths of labradorite, prismatic crystals 

 of brown augite, and magnetite. 



The specimens were taken from two narrow dykes, cutting 

 the brown trachyte and parallel to the coast-line. 



(G. 9.) The specimen is from the margin of a dyke run- 

 ning at risfht andes to the coast-line. It shows a beautiful 

 chilled edge. In thin section it is seen to be a very fine 

 grained basaltic rock, with small phenocrysts of basic 

 labradorite in a ground-mass which is made up chiefly of 

 microlites of augite and plagioclase, with a considerable 

 amount of residual glass. One section shows a large pheno- 

 cryst of brown hornblende. 



Some of the lavas, which we have termed basalts, appear 

 to be types intermediate in character between trachytes and 

 normal basalts. While their exact systematic position 

 cannot be stated with certainty until chemical analyses 

 have been made, the mineral composition seems to indicate 

 that they belong to the trachy-dolerites of Abich (inter- 

 mediate rocks rich in potassium). Similar transition types 

 occur in the Azores, Teneriffe, and Ascension, i and also in 

 the Tristan d'Acunha group.^ Thus it would appear that 

 the trachy-dolerites are a constant feature in that line of 

 volcanic islands which sweeps across the South Atlantic. 



Tuffs. — The tuffs are greyish to brown in colour, with 

 rounded lapilli of volcanic rocks and broken crystals of 

 augite and felspar. In thin section they are seen to consist 

 mainly of fragments of glass. 



Limestone. — Specimen from a boulder. Microscopic ex- 

 amination shows that this rock consists of irregular grains 

 of granitic quartz, flakes of muscovite, and much fragmentary 

 organic material, conspicuous among which are spines and 

 plates of echinoderms: these various constituents are 

 embedded in a calcareous cementing material. The rock is 

 undoubtedly of terrigenous origin. 



1 Jour. ofGeol., v. p. 362. 



2 Trans. South African Phil. Soc. , xvi. 



