REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 125 



anchored, and on the shores of which the explorers collected specimens. Mr. Buchanan 

 observed that the hills here have the same structure as in the north, the eruptive sheets 

 appearing in the form of horizontal layers. The hills, however, are farther from the 

 coast, and a plain, broken only by Mount Campbell, extends from their base to Cape 

 Digby. Mr. Moseley has drawn attention to the glacial phenomena in the neigh- 

 bourhood of Betsy Cove. A series of roches moutonnees appeared to the north of 

 the port where the Challenger anchored. Betsy Cove and the neighbouring fjord of 

 Cascade Beach are two deep indentions opening into the great basin marked on the 

 Admiralty chart as Accessible Bay. Here there also opens a large valley, running far 

 into the country between two lofty chains of hills. The hills near this valley are rounded 

 on the summit, probably by glacial action. According to Moseley, the whole region 

 has been subjected to great denudation since it was glaciated, and the striaa and moraines 

 must consequently have been obliterated to a great extent. Everything seems to show 

 that the hills were cut out of a continuous sheet of volcanic rock, which formerly spread 

 over the whole region ; the summits are capped with basalt, showing columnar structure 

 in their sections. 



We shall first describe the compact coarser-grained specimens of basalt from Betsy 

 Cove. They are black, with an unequal fracture, formed by an aggregation of crystal- 

 line grains, amongst which yellowish patches of olivine, measuring half a centimetre, 

 plagioclase, and augite may be detected by the unaided eye. Under the microscope 

 large and sometimes very elongated microporphyritic sections of olivine appear. This 

 mineral is decomposed into a yellowish matter, not showing the usual green tint 

 of serpentine. The augite is transformed into a green substance, delessite or grengesite, 

 which also tends to replace the felspar ; it is found in every hollow, and surrounds all 

 the constituent minerals. The plagioclase crystals show an angle of extinction, which 

 classes them as anorthite or some very basic felspar. Large sections sometimes show at 

 the same time the albite and Carlsbad twins. The larger minerals are embedded in a 

 network of small plagioclase crystals, augitic microliths, and decomposed grains of olivine. 



Other specimens from the same locality are finer grained, and also distinguished by 

 a cellular structure. They are all greatly altered, some specimens so much so that they 

 appear earthy, are covered with oxide of iron, and are frequently red, with whitish 

 markings. The vesicles, from half a centimetre to a centimetre in diameter, are usually 

 lined with well -formed crystals of chabasite. Doleritic structure does not appear 

 when slices are examined microscopically ; microporphyritic structure is very rarely 

 seen, and, when observed, is due to a larger development of crystals of plagioclase. 

 These large sections of felspar are traversed by cracks, pervaded by a light-brown 

 substance, presenting the characters of silica in the state of chalcedony or opal. The 

 silica sometimes partly penetrates the mass of the felspar, but it is not found in this 

 mineral only, as it occurs in all the holes, where it assumes a purplish or brownish 



