166 PEOF. .T. B. HAEEISON OX EXTRANEOUS [vol. lxXV,. 



The minerals found in the residua of the high-level lime- 

 stones indicate either an origin from outbursts of acidic volcanic 

 rocks, such as trachyte, containing comparatively low propor- 

 tions of basic plagioclases and of ferromagnesian minerals, or, more 

 probably, are ejectamenta from sub-silicic rocks such as andesite or 

 basalt, in which case the basic constituents have been decomposed 

 by lateritization and weathering to form kaolin and limonite. 



The extraneous matter from the low -level corals proved. 

 to be subangular grains of green augite- — the characteristic one 

 of West Indian lavas, some angular fragments of hypersthene, 

 much plagioclase in sharply-angular fragments, a few specks of 

 ilmenite, some very finely-divided fragments of sanidine, and a 

 little glass in minute splinters, with much ferruginous kaolin and 

 quartz in the proportions of 32 - 8 and 13 per cent, respectively. 

 Some of the blebs of plagioclase contain minute ciystals of the 

 green augite. The larger grains of augite are characterized 

 by the usual inclusions, some being very minute octahedra of 

 magnetite ; while some of the blebs of felspar and fragments of 

 glass have inclusions of microlites or of air-bubbles. The largest 

 mineral fragments found measured in millimetres : — 



Felspar. Pyroxene. Glass. 



•70X-62 -75X-38 'lOx-lO 



The extraneous matter of the low -lev el beach -rock con- 

 tains a few minute prisms of green-brown hornblende, a very few r 

 minute grains of hypersthene, some flakes of chlorite, a little 

 plagioclase, many finely-divided fragments of sanidine, and a few 

 minute grains of ilmenite ; with kaolin and grains of quartz in 

 greater relative quantities — 52 and 20 per cent, respectively — than 

 they are in the coral residua. The largest mineral fragments 

 found measured in millimetres : — 



Felspar. Pyroxene. Glass. 



•60X-25 -53X-30 --iOx^O 



The mean sizes of the larger mineral fragments found in the 

 limestone were as follows (in millimetres) : — 











Kaolin and 





Felsp>ar. 



Pyroxene. 



Glass. 



Limonite. 



High-level reef -corals... 



•46X-38 



•14X'08 



•IOX'10 



•07X-04 



High-level beach-rock... 



■34X-25 



•13X-09 



•20X-13 



•06X-04 



Low-level corals 



•36X-25 



•25X-16 



•llX'07 



•06X-04 



Low-level beach-rock . . . 



•21X'1(3 



•30X-18 



•27X-20 



•05X-04 



The majority by far of the fragments of minerals and splinters 

 of volcanic glass in every residue examined were, however, less than 

 "18 mm. ( TT7) th of an inch) in diameter. It is noteworthy that 

 the largest fragments of minerals and glass were found in the 

 fossil corals, while by far the highest proportion of minute specks 

 of minerals, and especially specks of glass, were present in the 

 beach-rocks. 



The analyses and the microscopical examinations of the residues, 



