Yol. 57.] ROCKS FROM BUFTON PIKE. 33 



now lie, or to solution by hydro thermal causes, seems to be an open 

 question ; but it appears more probable that it may be attributed to 

 a slowly dissolving solfataric action. This view is favoured by 

 the presence of a small amount of opal-silica, which has been 

 demonstrated by the staining of certain spots in an uncovered 

 section of the same rock by treatment with malachite-green, and 

 the permanence of the stains after the section had been washed in 

 hot alcohol. The spongy-looking fragments, above mentioned, have 

 a very fine granular structure which renders them readily distin- 

 guishable from the other porphyritic crystals in this rock. Car- 

 bonates are present to a trifling extent: an uncovered section, 

 Avhen treated with dilute hydrochloric acid, effervescing chiefly 

 around the margins of included crystals and fragments. 



Here and there clear, colourless streaks, which are rarely 

 persistent for more than short distances, may be seen to lie in the 

 o-roundmass. In some cases they apparently constitute bands 

 following the general direction of the fluxion-banding. They 

 consist for the most part of quartz, with slightly translucent 

 brownish or reddish-brown crystals; also opaque white pseudomorphs 

 with approximately square sections, which may possibly have been 

 spinel or garnet, but it is difficult to form any decided opinion upon 

 this point. There are also small opaque black octahedra, which are 

 no doubt minute crystals of magnetite. Possibly a small amount 

 of ilmenite may have been present; but, if so, it is now altered to 

 leucoxene, and evidence upon this subject is unsatisfactory : the 

 analysis of the rock shows, however, a small amount of titanic acid, 

 only 0-45 per cent. 



The rock occasionally contains a few crystals, which in ordinary 

 light are clear, colourless, and in some sections appear as elongated 

 prisms with sharply-defined transverse cleavages ; while in what 

 appear to be basal sections of the same mineral the^ cleavages 

 intersect at right-angles. I think that this mineral is possibly 

 scapolite. It is, however, only an exceptional accessory. 



Taking the whole of the evidence afforded by the microscopic 

 examination of this rock, it may be regarded as a tufaceous 

 rhy elite. Its very exceptional spotted groundmass is due to the 

 inclusion of volcanic ejectamenta, mainly as a rather coarse dust, con- 

 sisting chiefly of augitic, mingled with felspathic material, which 

 has, to a considerable extent, undergone alteration. Small fragments 

 of other rock, possibly of an andesitic character, but too much altered 

 to permit of precise determination, may also be detected. These 

 lapilli constitute a very small proportion of the rock. 



The great alteration of many of the porphyritic felspars ; the 

 corrosion of what appear to be felspars derived from an earlier 

 source ; the pseudomorphic nature of many of the smaller crystals 

 in the rock ; the presence of a small amount of opal-silica, indi- 

 cated by the staining produced by malachite-green ; and the general 

 appearance of a disintegration of crystals by solution, rather than 

 by fusion ; all seem to point to solfataric action as the chief 

 cause of the changes which this rock has undergone. 

 Q. J. a. S. No. 225. i> 



