60 REV. E. BAK.0N — GEOLOGICAL NOTES [Feb. 1 89 5, 



fissure-injections. Very thick folia of hornblende, or hornblende 

 with a little felspar, are also noteworthy. 



Otber rocks occurring in this province of Antsihanaka are : — 



Crystalline limestone and serpentine, near Ambatondrazaka, 



the capital of the province ; 

 Hornblende-pyroxene-granulite (123 : ), pyroxenite (Hunt) (113), 



and nepheline-basalt (124), near Amparafaravola, a village on 



the west side of Lake Alaotra ; 

 Limburgite (114, 115), near Mount Ambongobe, south-east of the 



village of Ambatondrazaka ; 

 Also a kind of trap-granulite (236). 



The country between the Antsihanaka province and the east 

 coast is of a very mountainous character and is largely occupied by 

 forest, the rock being so thickly covered over with soil and decayed 

 vegetation that there are comparatively few good exposures. Suffi- 

 cient evidence is, however, forthcoming to show that the rock 

 is chiefly gneiss. About 6 or 8 miles east of the village of 

 Ankaitomboka (long. 49° E., lat. 17° 20' S.), on the eastern 

 boundary of the Antsihanaka province, a form of trap-granulite 

 without a rhombic pyroxene occurs (264). About 5 miles east 

 of the same village another form of the same rock occurs (283), 

 but this time without garnet. Some of the trap-granulites might 

 perhaps be styled norites, but their structure is thoroughly gra- 

 nulitic. Oli vine-basalt (329) occurs also to a small extent in this 

 neighbourhood. Between the villages of Tsarasambo and Salangina 

 (about 25 miles east of Fenoarivo) there is an exposure of garnet- 

 rock, consisting of red garnet and ordinary green hornblende, 

 though some portions of it are composed almost exclusively of 

 garnet. It may be an extreme form of trap-granulite. There are 

 also, especially as one nears the sea, several dykes of dolerite, sub- 

 ophitic in texture, which run in a generally north-and-south 

 direction. It matters not from what point in the interior the 

 country is traversed to the east coast, these dolerite-dykes are invari- 

 ably met with. On the road, for instance, from the port of Tamatave 

 to the capital there are at least six or eight such dykes, the most 

 westerly of which is about 45 miles, and the most easterly about 

 8 miles, from the east coast. This fact, along with the almost 

 unvarying similarity in mineralogical and textural character of the 

 rock composing these dykes, points to the conclusion that they 

 run continuously for great distances along the lower eastern slopes 

 of the island. If the freshness of their condition be any criterion 

 of their age, they belong to a comparatively recent geological period. 



III. The East Coast. 



Perhaps the most notable fact in regard to the geological structure 

 of the northern half of the east coast (as also, so far as I can learn, 



1 The numbers in parentheses throughout this paper refer to the microscope- 

 slides in my collection. 



