88 ME. H. N. EIDLEY ON THE 



basalt tuffs and scoriaceous rock from the top of a lava-flow 

 were obtained. Passing west from Eat Island the basalt is almost 

 entirely submerged till Platform Island (San Jose). This island 

 is about 76 feet in height and crowned with reef -rock and some 

 indurated sand-rock resembling quartzite. There are two smaller 

 detached rocks of columnar basalt on the N.W. side. 



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

 with pebbles, bare at very low tides, the remains apparently of a 

 peninsula of which Morro do Chapeo, a small rock about 12 feet 

 in height capped with raised reef, is all that is left above sea-level. 

 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 appears in the western extremity 

 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 sj)ots, as at Sponge Bay, 

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

 times of immense size. Near Tangle Eock they were much 

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



Dylces. — 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 they traverse beds of phouolitic tuflls, and 

 some are as much as three feet in thickness ; they are often 

 transversely columnar, and curved and even branched. Similar 

 dykes occur in Leao Bay, here running north, and on the north 

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

 traversing beds of scoria. They are here of considerable height, 



