190 MISS G. L. ELLES ON THE RELATION OF THE [May 1909, 



VII. Petbological Notes on the Lavas. 



The Conway Mountain lavas are all classed as rhyolites in the 

 present paper, but they are by no means of uniform type and differ 

 as regards both their macroscopic and their microscopic characters. 

 Taken as a "whole, they agree in the general paucity of phenocrysts, 

 which are difficult to detect in hand-specimens ; but under the 

 microscope the phenocrysts are more conspicuous, and are usually 

 orthoclase or oligoclase. Ferromagnesian minerals are very rare, 

 although in one rock pseudomorphs after idiomorphic augite have 

 been observed. 



With regard to the Nodular Rhyolites, there is nothing to add 

 to the description of these rocks already given by Prof. Bonney, 

 Prof. Cole, and Mr. Harker (op. cit.). 



As regards the other rhyolites, the differences come out most 

 conspicuously in the characters of the ground-mass : this may 

 be said to be of two types — (1) confusedly macrocrystalline, and 

 (2) trachytic. 



Broadly speaking, the first is characteristic of the Upper Brecciated 

 Rhyolites, while the second is commonest among the members of 

 the Lower Banded Group. These last certainly are highly felspathic 

 rocks, with a good deal of quartz and little or no ferromagnesian 

 mineral ; flow-banding is beautifully shown by the trend of the 

 felspar-microlites. On the grounds of this trachytic structure there 

 may be objections to their inclusion among the rhyolites, but 

 although they were perhaps originally somewhat more basic than 

 the higher lavas of the series, it is scarcely possible, as Mr. Harker 

 has pointed out to me, to term them andesites, when the paucity 

 of ferromagnesian constituent is taken into account. 



Micropcecilitic structure is common in rocks of the first type, 

 though it also occurs in patches in rocks of the second type ; it is 

 evidently a secondary structure, and would seem to be a frequent 

 accompaniment of the processes of silicification and devitrification. 

 In some cases, the quartz enclosing the felspar forms a more or less 

 regular mosaic resembling that of vein-quartz. 



Slide I. Dark-grey banded rhyolite, New Quarry, northern 

 slope of Conway Mountain (western end). — This slide, which is cut 

 at right angles to the direction of flow, shows phenocrysts of ortho- 

 clase and oligoclase in a trachytic ground-mass. 



The phenocrysts are unusually abundant, and show a tendency to 

 occur in groups ; they are all more or less altered, some being 

 sericitized, while others are replaced by calcite or dolomite and 

 another brightly-polarizing mineral (which Mr. Fearnsides considers 

 is an epidote). Flow-banding is indicated by the felspar-microlites ; 

 and there is in places a tendency in the direction of micropoecilitic 

 structure, but it is not at all pronounced. 



Slide II. Light-grey banded rhyolite, same locality. — This 

 slide is cut parallel to the flow, which is clearly seen. The rock is 



