Vol. 51.] Or A JOURNEY IN MADAGASCAR. 65 



the rock is generally 10° to 15° N.W., and the actual thickness of 

 the limestone must be very considerable. The River Lokia, there- 

 fore, which runs from west to east, is probably situated where the 

 northern coast of the island lay in Jurassic times, and may be taken 

 as the present northerly limit of the outcrop of the gneiss and allied 

 rocks that form the greater part of the eastern half of the island. 

 I say ' Jurassic times,' because, judging from the fossil contents, 

 which have been examined by Mr. R. E. Newton, F.G.S., there can 

 be no question as to the age of these sandstones and limestones 

 (though the latter may be in part Cretaceous), for although many of 

 the fossils have a wide range of distribution, some of them can be 

 classified with certainty as belonging to the Jurassic series of rocks. 

 (See his paper, following the present one.) 1 



Between lat. 12° 45' S., where the mountain-range very abruptly 

 ends, and Diego Suarez Bay (about lat. 12° 15' S.), the chief feature 

 of interest is the mountain of Ambohitra, which is situated about 

 one-third way across the island, being nearer the west than the east 

 coast. The position of its highest point is about long. 49° 1 0' E., 

 lat. 12° 40' S. It runs in a direction north and south for a 

 distance of 10 or 12 miles and is probably about 5000 feet high, the 

 highest point I attained being 4100. It is an extinct volcano, and 

 has poured out lava which has flooded the low-lying country from 

 sea to sea, and from Diego Suarez Bay in the north to about 

 lat. 12° 35' on the eastern, and about 12° 50' on the western side 

 of the island to the south. 2 The area thus covered by the lava is 

 probably no less than 1200 square miles. One small crater, now 

 occupied by a lake, I discovered high up on the mountain, and 

 another and larger lake, probably also a crater, is said to exist near 

 the summit. The rock, from whatever locality it be taken, whether 

 from the town of Diego, on the southern side of Diego Suarez Bay, 

 or any other part of the lava-bed, or from any part of the mountain 

 itself, is remarkably homogeneous in character : it is an olivine- 

 basalt of a dark grey colour, the olivine-crystals being largely 

 altered to serpentine. The upper part of the bed, where not 

 decomposed into soil, is often very cellular, but for the most part 

 the rock is compact. I nowhere found any traces of lapilli or ashes. 

 Some of the numerous streams issuing from the mountain have 

 excavated valleys through the lava to a depth of 200 or 300 feet. 

 One of these streams (the Mariarano), which flows into the Bay of 

 Rodo on the north-east coast (lat. 12° 38' S.), has cut through the 

 lava-bed and into the underlying rock. Around the base of Ambo- 

 hitra a goodly number (probably a score or two) of parasitic cones 

 exist, some of which show breached craters, but none of which 

 are in a good state of preservation. One of these (south-west of 

 Ambohitra) I examined very cursorily. Some of the layers on the 



1 [The sandstone fossils alluded to here are from the Antankarana province 

 on the north-western boundary of the central hill-range. — B.. B. N.] 



2 Immediately to the south-east of this bay, however, there is a mountain- 

 mass, covering an area of probably 100 square miles, which stands out high 

 above the lava-bed ; of this I shall speak by-and-by. 



Q.J.G.S. No. 201. p 



