118 THE VOYAGE OF H.M.S. CHALLENGER. 



1200 feet, are designated Table Mountain. Eoss discovered at the top an oval crater- 

 like depression, the long axis of which measured about 100 feet. These heights are 

 formed, like the others already described, of horizontal basaltic layers, but in this case 

 they do not make up the whole mass. Pre-existing hillocks of pale grey rock were 

 surrounded by the lava-flows, contrasting in colour with the black encasing rock. 

 When speaking of Greenland Harbour we shall describe with greater detail the relations 

 and the aspect of these masses surrounded by the basalt, as in both localities the same 

 state of things occurs, and the observations recorded by Mr. Buchanan in that region 

 are more explicit from the present point of view than those available for Table 

 Mountain. Here we limit ourselves to the consideration of the most interesting rocks 

 of the latter region. According to Mr. Buchanan, the basalt assumes a columnar 

 structure and contains great nodules of olivine. The summit of the hill is covered with 

 fragments of basalt which are broken prisms. 



All the specimens which we have examined from Table Mountain belong to the 

 basaltic series. We will describe them in the order in which they were collected by 

 Mr. Buchanan when he climbed the hill. 



A doleritic rock is first found at the height of about 500 feet above the sea. 

 This appears compact to the naked eye, but crystalline grains may be distinguished. 

 Very small vesicles are scattered through the mass, which is furrowed by long 

 cavities from one to two centimetres in diameter lined with clearly defined crystals 

 of chabasite. Red oxide of iron penetrates the rock in certain points. Microscopic 

 examination shows that this dolerite is entirely impregnated with a greenish secondary 

 mineral. The crystals of olivine which formerly existed are now only recognisable by 

 the outlines of the sections ; the interior is entirely converted into this green matter. 

 The plagioclase also is so much altered that it no longer shows polysynthetic striation 

 between crossed nicols ; it is so penetrated by delessite that only a very narrow 

 frame of felspar surrounds the sections. The augite appears to have resisted decom- 

 position better, as a rule ; reddish sections of it, giving the optical reactions of this 

 pyroxene, are to be seen enclosed between the plagioclastic lamella?. It is sometimes 

 partly covered by an opaque brownish matter which surrounds and accentuates the 

 crystalline outlines. This opaque matter is formed of elongated or slightly-curved 

 black filaments resembling trichites or crystallites of magnetite. 



At the height of 1000 feet, about 10 feet below the terminal plateau, Mr. Buchanan 

 found a specimen of a granular rock, in which crystals of felspar could be distinguished 

 by the unaided eye ; its colour is light green by alteration, and its fracture irregular. 

 Microscopically it appears to be a much altered dolerite. As in the preceding rock, 

 olivine has almost entirely disappeared, but plagioclase in large lamelke and augite have 

 better resisted decomposition. Spherules of chalcedony and chabasite are developed in 

 the pores. Silica has also penetrated the felspar, and the plagioclase thus assumes 



