Vol. 5 I-] GEOLOGICAL NOTES OE A JUURNEI IN MADAGASCAR. 59 



logical composition. For many square miles (how many I cannot 

 say) in the neighbourhood of the village of Sahatavy, about 20 miles 

 west of Eenoarivo (east coast), the rock assumes a very distinctly 

 banded structure, consisting of layers of white felspar (orthoclase 

 and plagioclase), quartz, and dark mica, and is also somewhat 

 garnetiferous. This is the most distinctly and beautifully banded 

 gneiss that I have seen anywhere in the island. The prevailing 

 direction of the strike of the rock between the capital and the 

 Antsihanaka province, including the latter (the limits of which 

 province may be given as about long. 48° 20' E. to long. 49° E., and 

 lat. 17° S. to lat. 18° S.), is north-west and south-east, or north- 

 north-west and south-south-east, although the general strike of 

 gneiss in the island is a few degrees east of north and west of 

 south. When I speak of the strike of the rock, however, I mean 

 the strike of the foliation. As far as my observations of the 

 Madagascar gneiss and its allied rocks go, the foliation appears to 

 coincide with bedding — at any rate with the chief divisional 

 planes. East of the province of Antsihanaka, however, as far 

 as the coast, the rocks assume the prevailing trend of a few degrees 

 east of north and west of south. 



In the Antsihanaka province itself another type of rock comes 

 into great prominence, though not to the exclusion of the gneiss : 

 this is olivine-norite. (See the map facing this page.) It occupies 

 a very large area (not improbably at least 200 square miles), though 

 its precise boundaries are unknown. It rises into numerous fairly 

 big hills, and is doubtless the rock underlying Lake Alaotra, 1 as 

 it is found abundantly on both sides of the water. The grains of 

 olivine in this rock are surrounded by a shell of hypersthene, out- 

 side which is a layer of actinolite. But norite without olivine also 

 occurs. Occasionally the norite contains almandine-garnet. It is 

 generally, though not always, fairly well foliated. The strike of 

 this norite is also north-west and south-east, or north-north-west and 

 south-south-east. Numerous patches of amorphous quartz, often 

 milky-white in colour, make their appearance in the norite and crop 

 out on the surface. 



Coarse hypersthene-rock occurs in a massif or immense boss on the 

 west side of Lake Alaotra. At the north and north-east end of the 

 lake there is a very large exposure of quartz-magnetite rock, covering 

 an area of many square miles. In some portions of it the magnetite 

 appears to be almost or quite as abundant as the quartz. Occa- 

 sionally, as between the villages of Imerimandroso and Tsarahone- 

 nana, the magnetite is replaced by thin layers of a golden-looking 

 mineral, which proves to be a ferruginous actinolite. In the 

 south-east of Antsihanaka the gneiss is exceedingly coarsely 

 foliated, some of the folia swelling out to a foot or even a yard 

 in thickness. Amongst these pegmatite and graphic granite occa- 

 sionally occur, as, for instance, at the village of Mangalaza. These 

 are apparently part and parcel of the crystalline rock itself, and not 



1 For an account of tins extensive ancient lake, see my former paper in Quart. 

 Journ. Geol. Soc. vol. xlv. (1889) p. 306. 



