ROCK SPECIMENS FROM THE ISLAND OF SOCOTRA. 
293 
area of great antiquity and of a land surface which may have been an “ ark of' refuge ” 
to a terrestrial fauna and flora from one of the very earliest periods of this world’s 
history. 
Description of Plates. 
PLATE 6. 
Sketch map of Socotra. 
The information at the Author’s disposal has not enabled him to attempt to 
lay down the geological boundaries with any accuracy or to insert minor features, 
such as dykes, &c. It will be understood that the rock in the parts left blank 
(except where alluvial plains occur) is limestone of approximately middle tertiary 
age. Only the more important names are placed on the map. 
PLATE 7. 
1. Labrador diorite from base of Azalin (p. 281) X 20. 
>+ (a.) Labrador felspar, in parts rather decomposed ; the striping is indicated as 
when a polarizing apparatus is used. This has consolidated before the 
hornblende. 
(c.) The sap-green hornblende, within which is a crystal of iron oxide, probably 
ilmenite, and the pyroxenic constituent (b) of the text. Since writing this I 
have noticed that one of the two vertical sections of the mineral extinguishes 
when the cleavage lines coincide with the vibration planes, and is rather 
dichroic, giving reddish and greenish tints, according as the vibration of the 
polarized beam is perpendicular to or parallel with the cleavage lines of 
the mineral. The cross sections, though much resembling augite, are rather 
abnormal in some respects. Thus, though the evidence is insufficient for 
certainty, I think it possible that we have here the rhombic pyroxene 
(hypersthene ?) lately described by Mr. Whitman Cross (‘ Amer. Jour. 
Science,’ Feb., 1883) and Mr. Teall ( £ Geol. Mag.,’ Dec. ii., vol. x., p. 145). 
2. Vein granite from the slopes above Hadibu (p. 283) X 20. 
The figure contains two crystals, selected from different parts of the slide, 
to represent those crystals which while resembling hornblende in cleavage have 
optical characteristics agreeing with tourmaline. They are extremely dichroic, 
changing from a clear sap-green to a blue-black. 
