REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 121 



Cumberland Bay is the first important indentation of the coast to the south-west of 

 Christmas Harbour, but as neither the Challenger nor the "Gazelle" Expeditions collected 

 rock specimens from this deep and narrow fjord, our geological knowledge of it is limited 

 to the observations of Ross. He states that a hill 300 to 400 feet in height, formed of a 

 basaltic conglomerate and terminating in a crater, stands at the head of the bay. Veins 

 of an amphibolic rock are injected through the mass. On the south there is a bed of 

 carbonaceous matter 1 feet wide and 1 foot thick, covered by an amygdaloidal rock. A 

 little farther south another bed of coal, two feet thick, appears. The schistoid rocks at 

 the north of Cumberland Bay show impressions oifucus. Ross describes the rocks of the 

 bay as " trap," an expression which may apply to basalt or to more or less amygdaloidal 

 dolerite. Buchanan observed that, although geodiferous rocks are very common in this 

 part of the island, the nature of the geodes differs in various localities. At Cumber- 

 land Bay the cavities are filled with quartz crystals ; at Howe's Island, of which we 

 shall speak presently, chalcedony and agate predominate ; on the other hand, the 

 amygdaloidal cavities of the basalt are lined or filled chiefly with zeolites. To sum 

 up, quartz crystals seem to be confined to Cumberland Bay ; zeolites are chiefly 

 found at Christmas Harbour, while Mr. Buchanan observed none at Howe's Island or 

 Betsy Cove. 



The bay of Rhodes is shut in between Bismarck Peninsula and the large island of 

 Prince Adalbert, and there amygdaloidal basalts occur, some specimens of which we 

 have examined. The cavities are filled with chabasite, the rocks themselves much 

 altered, of a greyish colour, and entirely impregnated with zeolites, the constituent 

 minerals not being apparent to the naked eye. Microscopic examination shows that 

 these fine-grained rocks are composed of plagioclastic lamella?, augite, magnetite, and 

 several black opaque elements ; but they contain little or no olivine. The microscopic 

 vesicles are filled with closely-packed grains of chabasite. 



Professor Roth mentions the occurrence at Port Marie in Rhodes Bay, Prince Adalbert 

 Island, of some amygdaloidal dolerites with nodules of quartz and chalcedony with 

 coatings of the same minerals showing impressions of the rhombohedron of calcite — \ R. 

 Calcite and zeolites are also observed in these rocks. At a height of 500 feet a 

 doleritic basalt decomposing into a ferruginous red clay is found. 



To the north, and almost at the entrance of Rhodes Bay, is Howe's Island, long 

 supposed to be a peninsula. It was visited by the Challenger naturalists, who found 

 amygdaloidal rocks in the north-east, the geodes of which were exclusively filled with 

 agate. The hill summits were strewn with these nodules, which remained in their 

 places after the containing rock had decomposed. 



Amongst the rocks of this island, those may be described which form the top of 



(PHTS. CHEM. CHALL. EXP. — PART VII. — 1889.) 16 



