Geology and Mineralogy of Madagascar. 385 



Close to the junctions the rock is compact ; the dominant variety, 

 however, is of a grey or bluish-grey colour and medium grain. 

 Irregular masses of a very coarse variety, characterized by long flat 

 prisms of pyroxene measuring an inch or more in length, occur here 

 and there in the dominant medium-grained rock. 



The principal constituents of the rock of the Whin Sill are plagio- 

 clase felspar, a pyroxene having certain special characters, and a 

 magnetic titaniferous iron-oxide. Apatite is invariably present in 

 very small quantity. Interstitial matter may generally be recognized, 

 also in very small quantity ; in the coarser varieties this interstitial 

 matter takes the form of micropegmatite. The accessory consti- 

 tuents are a rhombic pyroxene (bronzite), hornblende, biotite, 

 pyrite, and various green decomposition-products. Olivine has not 

 been detected by the author, either as a fresh mineral or in the con- 

 dition of a pseudomorph. The three principal constituents have 

 been isolated by means of the Sonstadt solution and a weak bar- 

 magnet, and separate analyses are given in the paper. The felspar, 

 if of one species, must be allied to andesine ; the opaque iron-ore is 

 strongly magnetic, and contains 33 per cent, of titanic acid. The 

 prevailing pyroxene develops a laminated structure, parallel to the 

 basal plane, by alteration. In composition it deviates in the most 

 marked manner from the rule laid down by Tschermak, Mg + Fe 

 being greater than Ca. Bulk-analyses of two specimens of the 

 Whin Sill were given, and one of these was calculated out by means 

 of the data supplied by the separate analyses of the three principal 

 constituents. 



The rock presents many points of resemblance to the continental 

 diabases. It comes nearest to certain Swedish diabases described 

 by Tornebohm and to the great masses of trap which occur in the 

 Mesozoic strata of the Atlantic border of North America. 



10. " A Critical and Descriptive List of the Oolitic Madreporaria 

 of the Boulonnais." By R. F. Tomes, Esq., F.G.S. 



11. " On the Structure and Affinities of the family Receptaculi- 

 tidee, including therein the genera Ischadites, Murch. (=Tetragonis, 

 Eichw.), Splicer ospongia, Pengelly, Acanthochonia, g. n., and Mecep- 

 taculites, Defr." By Dr. G. J. Hinde, F.G.S. 



12. " On the Pliocene Mammalian Fauna of the Val d'Arno." 

 By Dr. C. J. Forsyth Major. Communicated by Prof. W. Boyd 

 Dawkins, F.B.S., F.G.S. 



13. " Notes on the Geology and Mineralogy of Madagascar." By 

 Dr. G. W. Parker. Communicated by F. W. Budler, Esq., F.G.S. 



This paper commenced with a sketch of the physical geography of 

 the island of Madagascar. A central plateau from 4000 to 5000 feet 

 high occupies about half the island, rising above the lowlands that 

 skirt the coasts, and from this plateau rise in turn a number of 

 volcanic cones, the highest, Ankaratra, being 8950 feet above the 

 sea. With the exception of certain legends, there is no record of a 

 period when the volcanoes were active : two such legends were given. 



The known volcanic cones were enumerated. They extend from 



