JSotes on the Geology of Madagascar. 431 



2. " Notes on the Geology of Madagascar." By the Eev. R. 

 Barou. With an Appendix on some Fossils from Madagascar, by 

 R. Bullen IN'ewton, Esq., F.G.S. 



The central highlands of Madagascar consist of gneiss and other 

 crystalline rocks, the general strike of which is parallel with the 

 main axis of the island, and also, roughiy, with that of the crys- 

 talline rocks of the mainland. The gneiss is frequently hornblendic ; 

 its orthoclase is often pink ; triclinic felspar also occurs in places ; 

 biotite is the most common mica, but muscovite is not uncommon ; 

 magnetite is generally present, often in considerable quantities. 

 The gneiss is often decayed to great depths, forming a red soil, and 

 the loosened rock is deeply eaten into by streams. The harder 

 masses of gneiss, having resisted decay, stand out in blocks, and 

 have been mistaken for travelled boulders of glacial origin. Other 

 more or less crj^stalline rocks are mica-schists, chlorite-schists, crys- 

 talline limestone, quartzite (with which graphite is often associated), 

 and clay-slate. 



Bosses of intrusive granite rise through the gneiss. That east 

 of the capital contains porphyritic crystals of felspar which near the 

 uorthern edge of the granite are arranged roughly in a linear di- 

 rection ; here also the granite contains angular fragments of gneiss. 

 For the most part the granite of Madagascar is clearly intrusive, but 

 this may not always be the case. 



The volcanic rocks are of much interest. The highest mountains, 

 those lying to the S.W. of the capital, consist, in their higher parts, 

 of a mass of lava, for the most part basaltic, but with some sani- 

 dine-trachyte. The lava-streams are sometimes 25 miles long, and 

 successive flows, up to 50U feet in thickness, are exposed by the 

 valleys. From the great denudation which this area has undergone 

 and from the fact that no cones now remain, we may assume that 

 this volcanic series is of some antiquity. Of the newer volcanic 

 series there are numerous very perfect cones, dotting the surface of 

 the gneiss in many places. No active volcano now exists in the 

 island, but the occasional emission of carbonic-acid gas, the occur- 

 rence of numerous hot springs and deposits of siliceous sinter, and 

 the frequency of small earthquake-shocks, seem to show that 

 volcanic forces are only dormant and not entirely extinct. 



The ashes generally lie most thickly on the side of the cone 

 between north and west ; this is accounted for by the prevalence of 

 the south-east trade-winds. The volcanic areas are ranged roughly 

 in a linear direction, corresponding with the longer axis of the 

 island. 



Sedimentary rocks occur mainly on the western and southern 

 sides of the island. The relations of these to each other have not 

 yet been determined ; hut from the fossils (referred to the European 

 standard ) it seems that the following formations are represented : — 

 Eocene, Upper Cretaceous, jSTeocomian, Oxfordian, Lower Oolites, 

 Lias. Possibly some of the slaty beds may turn out to be Silurian 

 or Cambrian. The crystalline schists, &c., are probably, for the 



