Z74 



ME. F. RUTLEY ON EELSITIC LAVAS 



[May 1899, 



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finely divided particles of iron more highly 

 oxidized, which is confirmed by the chemical 

 analyses of the red and black obsidian. The 

 iron in the red variety is almost wholly sesqui- 

 oxide, while in the black obsidian there is a 

 slight excess of protoxide over that required to 

 form magnetite in combination with the sesqui- 

 oxide.' ^ In a paper published many years ago ^ 

 one of the reddish-brown obsidians from the 

 Yellowstone district was described, and it was 

 there mentioned that rocks of a similar nature 

 were to be met with in North Wales. This 

 devitrified obsidian from Bodlondeb is a good 

 example, what were possibly once red bands 

 being now represented by devitrified brown 

 glass. Such a change in colour would naturally 

 result from the action of water upon the ferric 

 oxide which originally gave its colour to the 

 glass, converting it into limonite. The section 

 contains porphyritic crystals of orthoclase 

 (sanidine), twinned on the Carlsbad type, but 

 much altered and kaolinized, and in some cases 

 containing brown devitrified glass. I am in- 

 clined to believe that in this section there is an 

 extremely small quantity of unaltered glass, an 

 observation which, if correct, tallies with that 

 of Prof. Bonney in the case of another rock in 

 the same neighbourhood.^ 



JS^. 7 is a deep greenish-grey rock con- 

 taining reddish-brown fragments, one of them ' 

 which projected from the surface of the specimen 

 having measured over ^ inch in length before 

 cutting. Under the microscope the section is 

 seen to consist to a great extent, if not almost 

 wholly, of fragments, the majority of them 

 being devitrified obsidian or rhyolite, some 

 colourless, others reddish-brown. Some opaque 

 white crystals and fragments, which in some 

 instances may be kaolin, in others leucoxene, 

 are present in this section. It is difficult to 

 decide whether the fragments are embedded in 

 a rhyolitic lava or whether the entire rock 

 consists of fragments. If the latter be the 

 case, the rock is essentially a rhyo lite-tuff. 

 ^ " The devitrification of some of the fragments 



is globulitic, and occasionally the globulites are 



I ' Obsidian ClifF,' U.S. G-eol. Surv. 7th Ann. Eep. 1885-86 [1888] p. 274. 

 ^ ' The Microscopic Characters of the Vitreous Eocks of Montana,' Quart. 

 Journ. Geol. Soc. toI. xxxvii (1881) p. 392. 



3 Quart. Journ. Geol. Soc. vol. xxxviii (1882) pp. 294-95. 









