208 



Dr. C. A. Raisin — Geology of Perim Island. 



definite rhombohedra, cements the grains and fills the vesicles. 

 The material is chiefly pumice, but some fragments are a glass 

 basalt. Broken crystals also occur of felspar, often plagioclase, of 

 green hornblende, and brown-stained augite or olivine. One or two 

 fragments exhibit peril tic cracks, and one, partly devitrified, has a 

 radiating structure. Above this tuff occurs a fine material described 

 as 'sand,' looser above, becoming harder below, but the character of 

 an imbedded nodule suggests that the deposit is a fine volcanic dust. 



A. 7 ft. 



V/VV 









B. 18 ft. 



C. 1 ft. 



Fig. 1. — Section in a quarry east of Balfe Point. 



A. Blocks of lava, described as imbedded in coarse calcareous sand. 



B. Fine material resembling ' sand,' contains concretionary felsitic nodules, and is 



probably a fine tracbytic asb. 



C. Wbite pumiceous tuff. 



The nodule is roughly cylindrical, hard, pale brownish, and consists 

 of felsitic fragments mostly sub-rotund, with a few angular chips 

 of minerals like those in the underlying tufi". The pieces are 

 crowded close together, cemented by an isotropic substance which is 

 mammillated in form, and in the broader spaces passes to chalcedony; 

 it is doubtless opaline silica. Thus originally fragments of the 

 viscid lava were thrown down which solidified in cryptocrystalline 

 condition, sometimes as a mere rim around a broken crystal ; and 

 a deposit of silica cemented the fragments. 



2. Surface Blades. — Specimens of these have been taken loose 

 from many localities, and others from the two sections examined 

 near the west of the island. In the section to north-east of Balfe 

 Point calcite fills veins and cracks, few and sparse in the lava beds 

 already described, but becoming abundant above, while the surface 

 deposit to a depth of 7 feet consists of blocks imbedded in fine 

 calcareous mud. These blocks are mostly a red or dark brown, 

 ropy, scoriaceous basalt, but one is composed of fragments and is 

 probably a brecciated lava. Two blocks sliced for the microscope 



