374 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



opaque crystals whicli are exported by the ton and utilized only in 

 the abrasive industry.' 



On the other hand, the list of gems is not definitely closed, for 

 from time to time, following the discovery of a new deposit, the 

 list is increased by the name of a mineral until then considered a 

 mere pebble, because though possessing some of the qualities enumer- 

 ated above, it has been lacking in transparency or in a pleasing 

 color. Madagascar furnishes some examples of this. One of the 

 most common constituents of the rocks which form the granite 

 mountains of all countries — potash feldspar — has been found in a 

 locality lost in the south (Itrongahy, about midway between Betroka 

 and Benenitra) in crystals of an admu-able limpidity, set off by a 

 yellow color as warm as it was unusual and which gave it the appear- 

 ance of golden beryl.^ Very near there, scattered through the soil, 

 were found some fragments of a species of mineral, the name of which 

 has never been heard by any of you, the "kornerupine." ^ 



Instead of forming grayish and opaque rods, as in the single deposit 

 in Greenland, where until recently it had only been found, it consti- 

 tutes a transparent, sea-green stone slightly recalling certain aqua- 

 marines but with an incomparably superior brilliancy. 



Up to the ])resent time no diamonds have been found in Madagas- 

 car, but nearly all the other gems occur there in great abundance. 



Many of the minerals are seen in their original matrix, others are 

 gathered in the alluvium resulting from the breaking up of their vem- 

 stone in place, while still others are a part of the alluvium accumulated 

 by the work of streams. 



The Grand He is made up principally of a basement of ancient 

 rocks, eruptive and metamorphic, ending abruptly on the eastern 

 side in high cliffs which are separated from the Indian Ocean by a 

 narrow plain, low and sandy, while toward the west the island ter- 

 minates in a way no less abrupt, serving as a buttress for some sedi- 

 mentary formations which come to an end in the Mozambique Chan- 

 nel. All the deposits of precious stones are located in the central 

 ridge and particularly on the high plateaus that crown the island. 



One of the principal attractions of a trip to Madagascar is the con- 

 trasts encountered at every step, contrasts due to nature, contrasts 



1 1 have shown that this corundum which abounds eastward from the meridian of Tananarive is formed 

 in mica schists in connection with granite. (Comptcs Rendus de I'Acad. dcs Sciences, vol. 154, 1912, 

 p. 797.) There were exported in 1911, 150 tons, and this quantity will without doubt be doubled in 1912. 



» Its hardness (6) is loss than that of the beryl, bkewiso the density (2.55 to 2.60) and also its refraction 

 (%= 1.5253, 7in»= 1.5248, np= 1.5197). 



' Kornerupine is a magnesium aluminium silicate. Once cut, it is distinguished from the aquamarine 

 as well as from the green andalusite of Brazil by its very great density (3.27) and especially by its refraction 

 (%= 1.0742, mm= 1.6733, np= 1.6613). (A. Lacroix, Comptes Rendus, vol.; 155, 1912, p. 675.) A variety 

 named prismatine has been found in the granulite of Saxe, but it is formed only of little grayish rods not 

 transparent. 



The feldspar of Itrongahy is accompanied by crystals of limpid diopsido, specimens of which of a bottle- 

 green color form a very pretty gem. Some violet zircons and green apatite might also be cut. 



