52 



THE VOYAGE OF H.M.S. CHALLENGER. 



III. 1"5307 grammes of substance, treated in a sealed tube with, hydrofluoric and 

 sulphuric acids, was titrated by a solution of potassium permanganate (1 c.c. = 0'005405 

 gramme ferrous oxide), of which 4 - 2 c.c. were required for oxidation. 



IV. 1*2723 grammes of substance fused, by Sipocz' method, with sodium-potassium 

 carbonate, gave - 0061 gramme of water. 



Percentage Composition of the Specimen. 



Silica, Si0 2 , . 

 Alumina, A1 2 3 , 

 Ferric Oxide, Fe 2 3 , 

 Ferrous Oxide, FeO, 

 Manganese, . 

 Lime, CaO, 

 Magnesia, MgO, 

 Soda, Na 2 0, . 

 Potash, K 2 0, . 

 Water, H 2 0, . 



101-16 



100-00 



Transitions of Augitic Trachyte into Amphibolic Trachyte, Andesite, 



and Ehyolite. 



The Green Mountain contains a great many rocks transitional between augitic 

 trachyte and neighbouring lithological types. 



We shall first consider amphibolic trachyte. This is a compact greenish grey rock, 

 in which crystals of sanidine may be discerned by the naked eye ; the surface is partly 

 covered with brilliant crystals of hornblende, to which reference will be made later. 

 The microscope reveals hornblende also amongst the essential constituents, which are 

 otherwise similar to those of augitic trachyte. The sections of hornblende show decided 

 pleochroism, the absorption being almost as intense as for biotite ; they are characterised by 

 the cleavage, but the planes of separation are not sharp ; on account of a slight deviation 

 in their direction, they appear as curved lines. Titanite may be noticed in the form 

 of inclusions in the amphibole. It seems very probable that free silica in the form of 

 quartz is a constituent of the ground-mass ; but, perhaps, this mineral is a secondary 

 product, as is very frequently the case in the rocks of Ascension, a great number of 

 which are silicified. 



This rock shows a very interesting peculiarity which has been already observed, 

 particularly by Vom Eath, on some blocks ejected from Vesuvius. The whitened and 

 softened appearance of the specimen indicates that it has been subjected to the action 



