88 



THE VOYAGE OF H.M.S. CHALLENGER. 



sections of augite and olivine stand out from the paste ; the former are zonary and 

 pleochroic : — 



v > 



pink. 



P > 

 yellowish pink. 



yellowish green. 



As at Tristan, some of the dykes of Inacessible show the alteration well known in 

 massive basalt when suddenly cooled : at the contact with the encasing rock it is 

 altered into a brilliant black vitreous coating a centimetre thick. This glassy modifica- 

 tion affords a beautiful example of devitrification by trichites of ilmenite, and shows a 

 tendency to perlitic structure. The glass itself is yellowish, and depolarises light at 

 certain points, usually near the edge of the small crystals or in the outer zone of the 

 vesicles, a phenomenon due to molecular tension. Small skeleton crystals of 

 plagioclase and augite microliths are abundant, but black dendritic structures pre- 

 dominate, resembling those described by Zirkel in tachylite. 



The following analysis of the black vitreous coating of one of these dykes produced 

 at the contact of the encasing rock has been made by Dr. Klement : — 



I. 1"0648 grammes of substance, dried at 110° C, and fused by Sipocz's method 

 with alkaline carbonates, gave - 007l gramme of water, 0'5120 of silica, - 2028 of 

 alumina, "102 8 of ferric oxide, "1003 of lime, and - 1035 of magnesium pyrophosphate. 



II. 1*1578 grammes of substance treated with hydrofluoric acid gave 0'1621 gramme 

 of sodium and potassium chlorides and - l718 gramme of potassium chloroplatinate. 



III. T0733 grammes of substance treated in a sealed tube with hydrofluoric and 

 sulphuric acid required 111 c.c. of potassium permanganate solution (1 c.c. =0 - 005405 

 gramme FeO) to oxidise the ferrous oxide. 



IV. 1'6478 grammes of substance treated with hydrofluoric acid gave - 0722 

 gramme of titanic acid. 



The non-altered basaltic mass adjacent to these vitreous black bands is filled with 

 arborescent trichites of ilmenite, which appear slightly brownish in transmitted light ; 



