"^Ol- 55'] FELSITIC LAVAS AND TUFPS IfEAK CONWAY. 173 



structure. It is charged witli a considerable amount of limonite 

 and chlorite, the latter mineral often forming fan-shaped groups 

 of crystals filling irregularly-shaped vesicles. It contains nume- 

 rous crystals and fragments of an opaque white mineral which 

 appears to be leucoxene, and, if so, represents, in most cases, 

 the complete alteration of ilmenite. The white patches are seen 

 in reflected light to be traversed by well-defined cleavages, which 

 sometimes intersect at 60°, and may indicate the rhombohedral 

 character of the original mineral. The limonite is pseudomorphous 

 after magnetite, or perhaps in some cases after pyrites. A few 

 fragments, possibly of porphyritic felspars, are now represented 

 partly by chlorite and partly by microcrystalline felsitic matte]-. 

 Microlites are numerous in the rock. They do not appear to 

 have any definite linear arrangement, and they mostly give 

 approximately straight extinction, but others show inclined 

 extinction : some of them are probably oligoclase. There also 

 appears to be a small amoaut of brown de vitrified glass present 

 in the section. The rock seems to have a tufaceous character, and 

 should be called a tufaceous rhyolite, not a rhyolite-tuff. 



No. 5 is a deep purplish-grey felsite, somewhat similar to the 

 preceding rock. On examining the section under the microscope the 

 rock, like No. 4, is seen to be a tufaceous rhyolite. The frag- 

 ments are to a large extent derived from rhyolites or devitrified 

 obsidian, but they vary, some being colourless, while others are of a 

 coifee-brown tint, occasionall}'^ showing delicate fluxion-banding. 

 There are also fragments of opaque rock, some pale, others dark 

 brown, but their precise nature is difficult to determine. The rock 

 may be regarded as a rhyolite which, prior to its solidification, 

 has enveloped fragments of rock, some vitreous, others lithoidal. 

 Among the fragments is a rounded piece of quartz composed of 

 four or five individual crystals differing in orientation, and one of 

 them is cut nearly at right angles to the optic axis. This aggregate 

 of quartz contains many liquid inclusions with bubbles : some of 

 the latter show spontaneous motion under a high power. 



No. 6. A deep greenish- grey to reddish-brown rock. This is a 

 remarkably fine example of a devitrified obsidian with corru- 

 gated fluxion-structure, emphasized by a banding of dark-brown with 

 colourless glass. Obsidians of comparatively recent date, consisting 

 of bands of glass which differ in colour, are well known, notably in 

 the Yellowstone district, where there are brownish-red obsidians 

 which, in thin section, are seen to consist of bands and threads of 

 oran2:e-red alternating with colourless glass. Concerning these 

 obsidians. Prof. Iddings remarks : — ' The transition of the yellow 

 and orange bands into black grains, the larger of which are recog- 

 nizable as magnetite, indicates that the former are made up of 



Note. — The lines B, C, and D on the map (p. 172) are merely intended to 

 suggest a possibility, and are mainly based upon inference. All the other 

 geological lines are copied from the 1-inch Geological Survey map (78 N.E.),- 

 published in 1852. Owing to the small scale of that map, it was scarcely 

 possible that it could represent the felsitic lavas in greater detail. 



