118 Cos?no Johns — Allotropic Forms of Silica. 



K 1632, cast of a Lithodomtis : raised beach, No. 1, north of Qosseir. 



H 1633a, ? Favia : Camp 4, Hamraweiu, near Qosseir. 



K 1640, (a) Prionastra3oid, indet., (6) Acanthastrcea sp., (c) iudet. : 



raised beach, No. 1, 520 feet high, north of Qosseir. 

 K 1664, an Orbicelloid : raised beach, 380 feet above sea-level, Wadi 



Shigeli. 

 H 1776, ? Mussa sp. : Abu Sha'ar Plateau. 

 H 1866a, ? Orbicella : Wadi Belih. 

 J 1826, Acanthastraa sp. : old beach, Camp 17, Wadi Barud. 



DESCRIPTION OF PLATES. 

 Plate VI. 

 Fig. 1. — Smilotrochtts s^. Abu Roash. No. 3795. «, seen from the side ; J, out- 

 line as seen from above. Nat. size. 

 2. — Favia htimei, n.sp. Miocene : Wadi Belih, Abu Sha'ar. No. H 1780. 



Part of surface of the corallum. x I5 diam. 

 3. — Orbicella meUahica, n.s\). Miocene: west flank of Jebel Mellaha. Part 



of the surface of 11 1S62. x 1^ diam. 

 4. — The same. Some corallites of 1818/'. x 2 diam. 

 5. — Orbicella schiveinfurthi (Felix). Miocene: west flank of Jebel Mellaha. 



Part of the surface of H 1862 (duplicate). Nat. size. 

 6. — 0. defrancel (Edwards & Haime). Miocene: west flank of Jebel Mallaha. 



II 1862. A few corallites from a transverse section, x 1;} diam. 

 7. — Stylina tetramera, n.?,^. Miocene: Abu Sha'ar. H 1818."' Part of the 



surface of corallum. x 2 diam. 

 8. — Leptastrcea barroni, n.sp. Miocene : surface of Abu Sha'ar Plateau. 



H 1864. a, part of surface ; b, another part of surface of same specimen ; 



e, part of internal section, x 2 diam. 



Plate VII. 

 Fig. 9. — Stylocmnia tubereulata, n.sp. Miocene: "Wadi Belik. H1814. a, the 



coral, seen from side ; nat. size, b, part of the surface ; x 2 diam. 

 ,, 10. — Prionastrcea li/onsi, n.s^. Miocene: Wadi Belih. H 1814«. a, part of 



worn surface ; b, part of transverse section. Nat. size. 

 ,, 11. — Solenaxtrcea elliptica, n.sp. Miocene: Abu Sha'ar. H 1818a. a, part 



of the surface ; x 2 diam. b, part of a transverse section ; x 4 diam. 

 ,, 12. — Solenastrcca chalcidicum {Voxsk.). Pleistocene: raised beach near Qosseir. 



J 2140. Part of the surface of a worn corallum. x 2 diam. 

 ,, 13. — Fiingia valida, Verrill. Pleistocene: Wadi Queh. K1560. Corallum 



from side. Nat. size. 

 ,, 14. — Goniastrcea pcctinata (Ehr.). Pleistocene: Jemsa, south of Jebel Zeit. 



J 1819. Part of the surface of the corallum. Nat. size. 



IV. — Allotropio Forms of Silica and their Significance as- 

 Constituents of Igneous Rocks. 



By Cosmo Johns, M.I.Mech.E., F.G.S. 



THAT silica appears as a constituent mineral of igneous rocks 

 in two distinct phases, viz. quartz and tridymite, has been 

 known for some time. The writer is not aware that any explanation 

 has been offered which would indicate the conditions determining 

 the appearance of one or the other in a cooling fused rock-mass. 

 He now proposes to describe certain experiments made with a view 

 to explain wby it is that though the free silica generally appears 

 as quartz, yet occasionally, as in certain trachytes, it crystallizes out 

 as tridymite. 



