34 ME. F. KUTLEY ON TTTFACEOUS EHYOLITIC [Feb. I9OI, 



According to the analysis made by Mr. Philip Holland (see p. 36), 

 the rock may be regarded as having the composition of a soda- 

 rhy oli te. Its tafaceous character, although scarcely perceptible iu 

 the hand-specimen, is sufficiently demonstrated under the microscope. 



The specimen was given to me many years ago by my former 

 colleague, Mr. J. G. Goodchild, who (I believe), from field-evidence, 

 considered that it was probably a tufaceous rock. That he was 

 right in this surmise is sufficiently clear. Whatever construction be 

 put upon it, it is certainly a very peculiar rock, and one upon which 

 a variety of discrepant opinions might be given. 



No. 2. Dufton Pike. — A very compact, pale bluish-grey to 

 brownish-grey rock, with a few darker grey and blackish-green 

 specks. The specimen has a somewhat uneven platy structure, 

 which on transverse fracture gives an irregularly ' stepped ' 

 aspect to the broken surface. 



Under the microscope the section appears, in ordinary transmitted 

 light, to consist of a pale yellowish to colourless substance : this 

 might be mistaken for a groundmass filled with small colourless 

 rods, which on further examination can be proved not to be 

 microlites. Numerous less translucent, granular-looking patches 

 occur throughout the nearly colourless matter. These are most 

 irregular in form, and their outlines are suggestive of little shreds 

 cut from an ordinary bath-sponge. This brown substance con- 

 stitutes apparently less than one-half of the rock. The nearly 

 colourless substance really consists of small fragments* of altered 

 felspar, while the smaller proportion of brown matter lies between 

 the often closely- packed fragments, and is the groundmass in which 

 they are embedded. Apart from the fragments, of which the rock 

 is mainly composed, a few crystals and fragments of felspars, 

 apparently oligoclase and andesine, are seen in this section, but 

 they are usually too much altered to admit of precise determination. 

 There are also some irregularly-shaped, opaque spots, visible here 

 and there, which in reflected light are seen to be snow-white. 

 They are doubtless leucoxene in most cases, since the analysis of 

 the rock shows the presence of 0-47 per cent, of titanic acid. If the 

 leucoxene be the result of an alteration of ilmenite, that alteration 

 must have been complete, since no trace of unaltered ilmenite is to 

 be seen in the section, nor do these white opaque bodies exhibit 

 any definite crystal-boundaries. 



The entire section is seen, both in transmitted and in reflected 

 light, to be traversed by an excessively delicate streaking, the lines 

 being too fine to be described as banding. They pass quite indis- 

 criminately through matrix and fragments, a circumstance which 

 shows that, if due to fluxion, that fluxion must have resulted 

 from reheating of the rock after its consolidation. It seems more 

 probable, however, that this streakiness has been superinduced by 

 soifataric action. Igneous fusion and the accompanying motion 

 of a fused mass would scarcely have permitted the delicate structure 

 of the altered felspar-fragments (later to be described) to have 

 escaped injury, if not disintegration, through any such movement, 

 and it seems more reasonable to attribute the delicate streaking 

 to hydrothermal agency. 



