chap. ni. Siliceous Sinter and Jasper. 53 



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 scoriaceous basalt are lined or filled with 

 fine, 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 jasper are included, the edges of 

 which insensibly blend into the surrounding mass ; 

 other patches occur having an intermediate character 

 between perfect 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 frag- 

 ments of the jasper, examined under the microscope, 

 seem to resemble 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 

 vesicles filled with chalcedony, like those in the scori- 

 aceous lava, — only on the supposition that a fluid, 

 probably the same fluid which deposited the chalcedony 

 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 pro- 

 duced 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 com- 



