August 14, 1879] 



NATURE 



371 



nearly 2,000 feet below the level of the country east and 

 west of it. Dante would have imagined it, not a ' circle ' 

 certainly, but a remnant of some region of the horrible 

 pit itself, which for a wise and gracious purpose had been 

 gently touched by the cooling breath of heaven. There 

 had evidently been a great commotion going on there in 

 the ages gone by ; for all the long valley was dotted with 

 rounded hills, giving it the look of boiling water or 

 bubbling pitch, which by some strange process had 

 suddenly become congealed." 



It will therefore be seen that igneous agency has been 

 a powerful factor in shaping the physical geography of 

 many portions of Madagascar ; in few places could that 

 agency have been present in a grander scale than in the 

 volcanic region of which Madagascar is the centre, 

 and the Comoro and Mascarene groups the extreme 

 points in either direction. An attempt has been made in 

 the accompanying sketch-map to show the prominent 

 features of the physical geography of the island already 

 noted. Probably closer examination would show that the 

 detached groups of extinct craters are all connected by 

 intermediate links, so as to form a continuous line of 

 igneous disturbance from the extreme northern point of 

 Madagascar to at least as far south as the 23rd parallel ; 

 and from the appearance of a line of hills seen at a dis- 

 tance south of this latitude, I am strongly inclined to 

 believe that there has been subterranean agency at work 

 even beyond the upper granitic plateaux, but no examina- 

 tion has yet been made of this southernmost region. 



With regard to the geology of Madagascar, but little is 

 at present known with any exactness, for no competent 

 geologist has yet made a systematic exploration of the 

 countr)'. There are, however, a few facts of a general 

 character which have been noted by various observers, 

 and these may be here collected together as a slight con- 

 tribution to a knowledge of this subject pending a 'more 

 complete and scientific treatment of it. 



As already mentioned, the elevated region which forms 

 so large a part of the central, northern, and eastern por- 

 tions of the island is largely composed of primary and 

 igneous rocks. Granite, gneiss, mica schist, and basalt 

 are present almost all through this high region, and 

 generally form the loftiest points in the country. In 

 a single hill there is often a considerable variety of rock 

 both in colour and texture : granite of various shades of 

 grey, red, and rose-colour, with the constituent parts both 

 fine and coarse. Veins of quartz, running both through 

 these and the clays by which they are overlaid, are often 

 met with, and very fine specimens of rock crystal are fre- 

 quently found. A hard whitish stone, which has some 

 resemblance t* the Yorkshire stone called Bramley Fall, 

 is u;ed in AntanJinarlvo for public buildings, as well as for 

 the native tombs. 



The lower hills, as well as the high moors, are usually 

 composed of a bright red clay, but below the surface this 

 often seems to pass into a light pink or white earth 

 resembling kaolin or china clay. This frequent change 

 of colour would lead one to infer .that atmospheric influ- 

 ences have something to do with' the difference between 

 the surface clay and that exposed in the numerous pre- 

 cipitous clefts which the rains excavate on the hill-sides. 

 In many places the material found amongst the rock 

 seems exactly like granite in its constituent parts, but 

 without the cementing elements, so that it can be cut 

 quite easily by a spade. The red clay ia sometimes 

 varied by a light brown clay on the hills, while the plains 

 and valleys are filled with rich alluvial clays, blue and 

 black in colour. In all these clays there is an apparently 

 total absence of all organic remains, either animal or 

 vegetable, so that it is not an easy task to determine their 

 geological age, and there is little sign of stratification, 

 although I have detected some appearance of this in the 

 focks, with tilting of the strata. 



In this elevated region there seem to be few, if any, 



sedimentary rocks of a more recent age than the primary 

 ones which are so prominent a feature of it. A soft 

 dark red stone is found in some places, but this appears 

 to be only a hardened clay. Columnar basalt has been 

 noticed in two or three places, as well as extensive beds of 

 volcanic ash, decomposed lava, scoria, and lava rock of all 

 varieties of hardness, in some of which crystals of olivine 

 are found in abundance. 



At one point, however, in the upper region of the 

 island a limestone deposit occurs. This is at Sirabd, to 

 the south-west of the Ank^ratra mountains, and from the 

 pits dug here most of the hme used for building in the 

 central province is procured. It has not yet been exa- 

 mined by any one with competent scientific knowledge, 

 but it appears to be a sulphate of lime, and is probably 

 only a local deposit and not a stratified rock, and most 

 likely is connected with the subterranean action so visible 

 all around the district. 



Clay slate is met with in the southern part of this ele- 

 vated region ; and in the Bdtsildo country a valuable slate, 

 suitable both for building and for writing upon, is found, 

 although it has not yet been worked to any extent. Ac- 

 cording to some accounts, greywacke or whinstone, silex, 

 and chert with chalcedony, are also met with in the 

 southern highlands. 



From certain of the facts above given, as well as from 

 other considerations, it appears highly probable that the 

 extensive elevated region of Madagascar is very ancient 

 land, and has most likely remained for many ages above 

 the waters of the Indian Ocean ; otherwise, some trace of 

 marine deposits would surely be found in some portion of 

 this great extent of country. I may, however, here note 

 the fact that there are in some places such rounded, 

 boulder-like masses of blue basalt rock, sometimes on the 

 surface and sometimes partially embedded in the soil, 

 that did these occur in the temperate zone, one would 

 certainly ascribe them to glacial action ; but the point 

 requires fuller investigation, and possibly some other 

 solution may be given to the rather puzzling inquiry sug- 

 gested. But in travelling to the north-west coast, as we 

 got near the sea-level, we met with boulders composed of 

 rock which certainly is not found in siUt anywhere near 

 the spot where these boulders occur, but has come from 

 far away in the interior. 



With regard to the lower region of Madagascar— the 

 extensive plains to the west and south of the island, as 

 well as the narrower extent of country on the east coast 

 — we have a little more definite information as to the 

 geology of some portions of it. This division of the 

 country is only as many hundreds of feet above the sea 

 as the granitic region is thousands of feet ; and there we 

 find not only deposits of the later Tertiary epochs, con- 

 taining fossils of animals but recently extinct, but also 

 fossils of the Secondary age. This fact was first pointed 

 out by M. Grandidier, who, in speaking of the south and 

 west portions of the country, says : '■'■ Nerinea ^r^A other 

 characteristic fossils of the Jurassic! formation which I 

 have there collected prove the existence of Secondaiy 

 strata, which cover a vast extent of this island" (/>«//. 

 df la Soc. de Gh,z-, ao"t. 'S/r, p. 88). In a later number 

 of the same publication (avril, 1872) he also speaks of an 

 extensive " terrain nummulitique parfaitemcnt charactdrisd 

 par des Neritina schnideHana, et pdtri de foraminifiires 

 appartenant aux genres Alveolina, Orbitoidcs, Triloculina, 

 &c." This is confirmed by the fossils discovered in the 

 south-west of Madagascar, in the upper part of the valley 

 of the St. Augustine river, by the Rev. J. Richardson in 

 1877. These occur in vast numbers, and from a drawing 

 he gives appear to belong to the Neocomian formation, 

 and are species of the genera Ammoniles, Terebratula, 

 Nerinea or Turritella, Einoceramus, and Rhynconella, 

 together with an Echinoderm. 



It is evident also that there are deposits of a much 

 later date than the above, for in the south-west of Mada- 



