



88 



MB. H. N. BIDLET ON THE 





basalt tuffs ar 

 were obtained. 



till Platform Island (S 



dt is almost 

 This island 







>wn 





indurated sand-rock resembling quartzite. There are two smaller 



W 



San Jose is connected with the mainland by a low ridge covered 



with 





Morro 



in 



Between this rock and the Main Island were a number of large 

 boulders of very hard basalt, containing " bombs " or balls of 

 olivine, enstatite, and augite, much resembling those of the 

 Laacher See, and very fresh in appearance. 



In the Main Island the basalt anoears in the western extremity 













i 











1 , 





























y 



or almost at sea-level and rises gradually towards the east hills, 

 and after forming the great plateau of the Central district passes 

 away into Cape Placelliere and Point Noir, in the western and 

 north-western ends of the isle. It occurs in the form of horizon- 

 tal layers at Tobacco Point, and round the lake, at Boldro and 

 elsewhere, and in some spots, especially at the last-mentioned place 

 it was not difficult to find the bands of scoriaceous and pumiceous 

 rock which had formerly formed the upper layer of a lava-flow, 

 which had again been covered by another and another lava-deposit. 

 The basalt here, again, was rich in olivine and augite, and the 

 prevailing rock was fine-grained, black, and compact. Besides 

 these layers were other masses, columnar in structure, the columns 

 being usually larger than those of the phonolite. They were very 

 well seen in Portuguese Bay. At other spots, as at Sponge Bay, 

 East Hills, and Tangle Bock, the basalt was in spheroids, some- 

 times of immense size. Near Tangle Rock they were much 

 decomposed and altered, and showed the presence of much iron. 

 Dykes. — Dykes of basalt occurred in several parts of the island. 

 At Sponge Bay they are numerous, running down from the East 

 Hills into the sea : here thev traverse beds of nhonolitic tuffs, and 















three 



are 



often 





transversely columnar, and curved and even branched. Similar 



running 







side of the island, near Cape Placelliere, where they run north-east 

 traversing beds of scoria. They are here of considerable height 







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