86 ME. H. N. RIDLEY ON THE 



7. COSCINODISCUS HETEKOPORTTS, JEhreiib. 



Diam. '14 mm. A very small central space, the markings largest 

 around the centre and at about f of the radius from it, beyond this 

 diminishing on a somewhat wide zone to the lower. 



8. C. MARGiNATUS, Uhrcnh. 

 Diam. '095 mm. Several. 



9. C. PEREORATUS, Ehreiib. 



Diam. '125 mm. Markings increasing slightly outwards to 3 

 in "01 mm. Small on a narrow band within the border. 



10. C. INTUMESCENS, Pailt. (?). 



Diam. '0875 mm. Rare. 



11. C. ocuLus-iRiDis, JEJirenb. 

 Diam. '2 mm. Few. 



12. C. ARGUS, ETirenh. 

 Diam. •1125 mm. Eare. 



13. C. EADiATUS, var. media, Grun. 

 Diam. '1075 mm. Few. 



14. C. ROBUSTA, Grev. 

 Diam. '0875 mm. Eare. 



15. C. ASTEROMPHALUS, Ehreuh. 

 DJara. '25 mm. Eare. 



GEOLOGY, 



Based on Petrological Notes hy Thomas Davies, F.Gr.S. 

 (Communicated by H. N. Eidlet, M.A., F.L.S.) 



The whole cluster of islands now above water only presents 

 two groups of igneous rocks, viz. phonolites and basalts, to 

 which must be added, by way of sedimentary rocks, some raised 

 coral-reefs and some sandstone formed of blown sand. 



The phonolite is confined to Sella Giueta, the Peak, Look-out 

 Hill, Tangle rock, Morro branco, the Central Knoll, and the 

 island known as tlie Frade ; but at Sponge Bay, on the south-east 

 side of the Main Island, are some beds of phouolitic tuffs traversed 

 by dykes. 



It is usually rudely columuar, with the columns generally 

 forming an angle with tlie horizon ; but on the Central Knoll they 



