62 KEV. E. BAEON GEOLOGICAL NOTES [Feb. 1 895, 



This glass occupies more than half of the rock and is crowded with 

 black trichites. At one place (about lat. 15° S., a mile or two from 

 the sea) I noticed some volcanic breccia. Here and there are to be 

 seen volcanic dykes cutting through the gneiss or granite on the 

 sea-shore, the rock from which sometimes proves to be the usual 

 subophitic dolerite and sometimes basalt. A doleritic dyke (376), 

 for instance, occurs immediately south of the small village of 

 Antseranambe, south of Antongil Bay. The rock at the margin of 

 this dyke contains a considerable amount of black tachylyte (278). 

 A basaltic dyke is to be seen 2 or 3 miles north of the village of 

 Manompana, opposite the island of Ste. Marie. In this instance 

 it cuts through dolerite, the basalt itself assuming a somewhat 

 andesitic habit (373). Another dyke of basalt (317) (approaching 

 to dolerite in texture), which may be called doleritic basalt, occurs 

 at the village of Tanjona in Antongil Bay. Doubtless other dykes 

 occur which are covered by the sand of the sea-shore. 



Although by far the greater part of the lava has consolidated in 

 the form of dolerite, there are one or two localities where it has 

 assumed a much finer grain and occurs as basalt. At Isoavinan- 

 driana, for instance, the capital of the province of Sahambavany 

 (lat. 14° 10' S.), many square miles of country are covered by a basalt 

 of exceedingly fine grain and of somewhat andesitic habit (275). 

 At the Biver Lokoho, again, some 3 or 4 miles south of Isoavinan- 

 driana, the rock is also a basalt (310) with andesitic habit. At 

 A_mboaniho (lat. 13° 25' S.), the capital of the Iharana province, 

 basalt also occurs (303-307). 



At this same town of Amboaniho, and for several miles north and 

 south of it, as also for some distance inland, occurs another type of 

 rock in great abundance, namely, felspar-porphyry (302, 465, 468). 

 Spreading, as it does, over a wide area, and forming very thick 

 masses, there can, I think, be no doubt as to its being an actual lava- 

 flow ; but from what source it has proceeded I cannot say. Nothing, 

 however, suggestive of volcanic cones is to be seen anywhere, so that 

 this flow also may possibly have issued from a fissure or fissures in the 

 crust. It varies more or less in character. Sometimes it is dark- 

 coloured, almost black, with phenocrysts of flesh-coloured crystals of 

 orthoclase. Its prevailing colour is, however, purplish. Occa- 

 sionally the rock is quite or almost free from phenocrysts of felspar, 

 when it may be called felsite. Felspar-porphyry, together with 

 felspar-porphyry breccia (300), both of a purplish colour, occur in a 

 hill 500 or 600 feet high, about 1 mile south of the village of 

 Amboaniho (lat. 13° 25' S.), the breccia lying above the porphyry, 

 and this again being underlain by somewhat altered basalt (304— 

 307), which forms the base of the hill. In other places the felspar- 

 porphyry becomes spherulitic (for instance, about 12 miles south of 

 Amboaniho), the spherules being small but very numerous. 



Felspar-porphyry occurs in small quantity also farther north, 

 namely, about 2 miles south of the village of Andravina (about 

 lat. 13° S.). 



Quartz-porphyry (453, 454), felsite? (449), and rocks with 



