Volcanic Islands visited during the Voyage of the Beagle, S*? 



sometimes adheres to the projecting plate-like veins, described in the last 

 paragra[)h. The jasper is of an ochre-yellow or red colour; it occurs in 

 large irregular masses, and sometimes in veins, both in the altered trachyte 

 and in an associated mass of scoriaceous basalt. The cells of the scoria- 

 ceous basalt are lined or filled with tine, concentric layers of chalcedony, 

 coated and studded with bright-red oxide of iron. In this rock, especially 

 in the rather more compact parts, irregular angular patches of the red jas- 

 per are included, the edges of which insensibly blend into the surrounding 

 mass; other patches occur having an intermediate character between per- 

 fect jasper and the ferruginous, decomposed basaltic base. In these patches, 

 and likewise in the large vein-like masses of jasper, there occur little 

 rounded cavities, of exactly the same size and form with the air-cells, which 

 in the scoriaceous basalt are filled and lined with layers of chalcedony. 

 Small fragments of the jasper, examined under the microscope, seem to re- 

 semble the chalcedony with its colouring matter not separated into layers, 

 but mingled in the siliceous paste, together with some impurities. I can 

 understand these facts, — namely, the blending of the jasper into the semi- 

 decomposed basalt, — its occurrence in angular patches, which clearly do 

 not occupy pre-existing hollows in the rock, — and its containing little vesi- 

 cles filled with chalcedony, like those in the scoriaceous lava, — only on the 

 supposition that a fluid, probably the same fluid which deposited the chal- 

 cedony in the air-cells, removed in those parts where there were no cavities, 

 the ingredients of the basaltic rock, and left in their place silica and iron, 

 and thus produced the jasper. In some specimens of silicified wood, I have 

 observed, that in the same manner as in the basalt, the solid parts were 

 converted into a dark- coloured homogeneous stone, whereas the cavities 

 formed by the larger sap-vessels (which may be compared with the air-vesi- 

 cles in the basaltic lava) and other irregular hollows, apparently produced 

 by decay, were filled with concentric layers of chalcedony ; in this case 

 there can be little doubt that the same fluid deposited the homogeneous 

 base and the chalcedonic layers. After these considerations, I cannot 

 doubt but that the jasper of Ascension may be viewed as a volcanic rock 

 silicified, in precisely the same sense as this term is applied to wood when 

 silicified : we are eqaally ignorant of the means by which every atom of 

 wood, whilst in a perfect state, is removed and replaced by atoms of silica, 

 as we are of the means by which the constituent parts of a volcanic rock 

 could be thus acted on*. I was led to the careful examination of these 

 rocks, and to the conclusion here given, from having heard the Rev. Pro- 

 fessor Henslow express a similar opinion regarding the origin in trap-rocks 

 of many chalcedonies and agates. Siliceous deposits seem to be very gene- 

 ral, if not of universal occurrence, in partially decomposed trachytic tuffs f; 



* " Beudant {Voyage en Hongi-ie, torn. iii. p. 502, 504) describes kidney- 

 shaped masses of jasper-opal, which either blend into the surrounding tra- 

 chytic conglomerate, or are imbedded in it like chalk-flints; and he com- 

 pares them with the fragments of opalized wood, which are abundant in 

 tliis same formation. Beudant, however, appears to have viewed the pro- 

 cess of their formation, rather as one of simple infiltration, than of mole- 

 cular exchange; but the presence of a concretion, wholly different from the 

 surrounding matter, if not formed in a pre-existing hollow, clearly seems to 

 me to require, either a molecular or mechanical displacement of the atoms, 

 which occupied the space afterwards filled by it. The jasper-opal of Hun- 

 gary passes into chalcedony, and therefore in this case, as in that of Ascen- 

 sion, jasper seems to be intimately related in origin with chalcedony." 



t " Beudant ( Toj/ffige Min. tom. iii. p. 507) enumerates cases in Hungary, 

 Germany, Central France, Italy, Greece, and Mexico." 



