212 prof. j. w. GREGOEr on the [May 1900, 



agglomerate and tuff, which in places also cover them. The typical 

 representatives of this lava are rhyolitic in aspect, earthy-looking, 

 vary from light red to pale brown, and show very conspicuous 

 fluxion-structure. 



Examined microscopically the rocks of this type appear to consist 

 of a brown, light green, and dark green glassy base, with numerous 

 opacite-granules and felspar-mieroliths, and large corroded pheno- 

 crysts of anorthoclase. The matrix is indeterminable microscopically, 

 but when treated with cold acid it becomes gelatinous and yields 

 abundant crystals of common salt ; we may, therefore, safely infer 

 that the glassy matrix is extremely rich in soda, and would probably 

 have yielded much nepheline had it crystallized. The rock repre- 

 sents the flows from the plug of solid lava that choked the main 

 vent. The specific gravity of the variety (No. 500) nearest the 

 lava of the central plug is 2-62, while in flows farther away 

 from the central core, at the Lewis Col and on the Teleki Ridge, 

 the specific gravity sinks to 2*5. 



The aspect of the rock is very variable ; as examples we may take 

 the following types : — 



No. 500, from north of the Lewis Col. Specific gravity 2*62. The 

 rock has a light-green matrix and an indefinite fluxion-structure ; 

 the phenocrysts of anorthoclase are large, but not crowded ; the 

 groundmass is dense from the abundant pale green microliths, which 

 Mr. Prior, from their optical characters, refers to aegyrine. 



No. 507, from the Lewis Col itself. Specific gravity 2-5. A coarse, 

 rhyolitic-looking, reddish lava, with well-developed fluxion-structure 

 round the anorthoclase-phenocrysts, which contain inclusions of the 

 glassy base. 



No. 508. Dark green, almost black rock, with small anorthoclase- 

 phenocrysts in a dense base of minute felspar-mieroliths, magnetite- 

 and aegyrine-grains. The felspars are often twinned on the Carlsbad 

 type. (PL XI, fig. 3.) 



No. 452 is a specimen from a flow of black lava similar to the 

 last, but including a bomb of a reddish rock. (The latter is closely 

 allied to No. 519, the reddish base probably, resulting from the 

 alteration of a basic glass.) The black lava contains many pheno- 

 crysts of anorthoclase showing well-developed twin lamellae. The 

 glassy base includes aegyrine and some ill-defined mineral, which is 

 probably aegyrine undergoing alteration into opacite. An imperfect 

 spherulitic structure occurs in places. Mr. Prior notes that some of 

 the phenocrysts are partly replaced by an aggregate of lath- shaped 

 felspars and pale-green augite, similar to the pseudomorphs after 

 felspar described by Brogger ! in grorudite. Mr. Prior also remarks 

 the resemblance of this rock to a specimen in the Natural History 

 Museum from the west side of the Val di Monastero, Costa di 

 Zichidi (Pantelleria). 



No. 519. One of the most interesting rocks in this kenyte-series 

 is a black porphyritic pitchstone, which occurs in Phonolite Cwm, 



1 ' Die Gesteine der Grorudit-Tinguait Serie ' Eruptivgest. des Kristiania- 

 gebietes, pt. i, in Vidensk. Skrift. pt. i (1894) No. 4, p. 15. 



