G8 THE VOYAGE OF H.M.S. CHALLENGER. 



or entirely filled by concentric layers of chalcedony coloured red by ferric oxide. 

 Irregular angular grains of red jasper, with an outline gradually becoming less definite 

 and passing into the surrounding mass, are found in the most compact parts of the 

 same rock ; there are also other grains which hold a position intermediate between 

 jasper and decomposed iron-coloured basalt. The jasperoid portions contain circular 

 cavities of exactly the same form as those occurring in scoriaceous basalt. Darwin 

 explains these facts by supposing a siliceous solution to have penetrated the rock after 

 the elimination of certain altered constituents. This interpretation appears very 

 natural, but with the specimens at our disposal, it would be rather difficult to 

 judge of its applicability ; we would require to see many more specimens than those 

 we have studied. With reference to these siliceous deposits, Darwin recalled the 

 frequency with which a similar action occurred amongst the altered trachytic tufas. 



Amongst the specimens collected by the Challenger, we have only found a few 

 fragments showing the siliceous infiltration to which reference has been made. Some 

 rocks from Riding School, and from the plain at the foot of Red Hill, show silicifica- 

 tion well. In proportion as silica develops in the rocks, the characters of the con- 

 stituent minerals become obscured, and various modifications of silicic acid invade the 

 ground-mass. 



One of the rocks from Red Hill is a true siliceous tufa, in which the original 

 constituents can hardly be distinguished. The rock is yellowish white to the naked 

 eye, decayed, so hard that steel will not scratch it, and milky fragments of quartz break 

 off from the mass. Under the microscope the ground-mass is seen to be nothing but 

 an aggregate of minute quartzy grains firmly compacted together. They are angular 

 and colourless, and behave between crossed nicols like the basis of certain quartziferous 

 porphyries. 



A volcanic glass almost entirely converted into silica is found at Riding School. 

 This rock is like a eurite, whitish in colour, very hard, homogeneous in texture, and has 

 a slightly scaly fracture. Microscopic preparations show a slightly vesicular vitreous 

 ground-mass. Chalcedony has formed in the pores and interstices of this glass, and in 

 some places the rock seems impregnated with imbricated crystals of tridymite. 



Siliceous Deposits of Organic Origin. 



The wide circular hollow, about half a mile in diameter, which surmounts the 

 " Crater of an old volcano," is not a crater according to Darwin. 1 The hollow is almost 

 filled with many-coloured layers of scoriae, cinders, and incoherent volcanic products. 

 The general appearance of the beds is saucer-shaped. They are all visible at the edge 

 of the hollow, where they show as a succession of variously-tinted rings, giving a 



1 Geol. Obs., pp. 47-49. 



