chap. v. Altered Tuff. 113 



gray, basaltic lavas in the mouths of craters standing in 

 the sea. It may be asked whether the heated water 

 within these craters has produced this singular change 

 in the small scoriaceous particles, and given to them 

 their translucent, resin-like fracture ? Or has the 

 associated lime played any part in this change ? I ask 

 these questions from having found at St. Jago, in the 

 Cape de Verde Islands, that where a great stream of 

 molten lava has flowed over a calcareous bottom into 

 the sea, the outermost film, which in other parts re- 

 sembles pitchstone, is changed, apparently by its 

 contact with the carbonate of lime, into a resin-like 

 substance, precisely like the best characterised speci- 

 mens of the tuff from this archipelago. 1 



To return to the two craters : one of them stands at 

 the distance of a league from the coast, the intervening 

 tract consisting of a calcareous tuff, apparently of sub- 

 marine origin. This crater consists of a circle of hills, 

 some of which stand quite detached, but all have a very 

 regular, qua-qua versal dip, at an inclination of between 

 thirty and forty degrees. The lower beds, to the thick- 

 ness of several hundred feet, consist of the resin-like 

 stone, with embedded fragments of lava. The upper 

 beds, which are between thirty and forty feet in thick- 

 ness, are composed of a thinly stratified, fine-grained, 

 harsh, friable, brown-coloured tuff, or peperino. 2 A 

 central mass without any stratification, which must 



1 The concretions containing lime, which I have described at 

 Ascension, as formed in a bed of ashes, present some degree of re- 

 semblance to this substance, but they have not a resinous fracture. 

 At St. Helena, also, I found veins of a somewhat similar, compact, but 

 non-resinous substance, occurring in a bed of pumiceous ashes, appa- 

 rently free from calcareous matter : in neither of these cases could 

 heat have acted. 



2 Those geologists who restrict the term of ' tuff to ashes of a 

 white colour, resulting from the attrition of feldspathic lavas, would 

 call these brown-coloured strata ' peperino.' 



