Vol.57-] AMONG THE MALVERN CAMBEIANS. 169 



detected under the microscope, are alone present. Small rounded 

 oavities filled with calcite, etc., which appear to be steam-vesicles, 

 may be occasionally detected (M 231, 258, 269) ; but I have never 

 seen the rock in the amygdaloidal condition mentioned by Phillips,^ 

 much less in the condition of ' pumice or scoriae ' described by 

 Symonds." Some of the rocks, even those of the freshest appear- 

 ance (M 249), effervesce freely with acid. Surface-oxidation has 

 almost invariably produced a thin brown crust ; indejjendently of 

 this, the more exposed portions of the rock weather green, dark 

 green, or brownish-green. Continued oxidation has produced in 

 some cases a rusty brown or orange colour, but very frequently the 

 gradual elimination of the green and red colouring-matter has 

 caused the rock to assume a light greenish, brownish, or yellowish- 

 grey colour. These greatly-weathered rocks often somewhat 

 resemble sandstones, for Avhich they appear to have been mistaken 

 (p. 180). The phenocrysts at first showing black to the eye, under 

 the influence of weathering lose their lustre, become dull green, 

 then brown, and finally disappear, leaving small brown cavities, 

 which have, I suspect, been regarded as vesicles. The rocks some- 

 times weather in a spheroidal manner. 



The specific gravity in eight of the fresher examples (see Table II, 

 p. 172) ranged from 2'61 to 2-82. A weathered example (M 117 h) 

 gave 2-56. 



Chemical analyses by Timins of three of the Bronsil masses 

 (probably M 247, 249, & 108) gave as silica-percentages 42'42, 

 40*00, and 39"25.^ Three analyses of two other basalts gave from 

 46'27 to 47*17 per cent, of silica ; and in two doubtful cases the 

 percentages were 41*85 and 43*53 (see Table IV, p. 176). The rocks 

 would accordingly appear to be of thoroughly basic, or of ultra- 

 basic, composition. 



Microscopic examination shows that although the original con- 

 dition is never retained, the prevailing types exhibit considerable 

 uniformity in structure and mineralogical composition. 



The only phenocrysts are pseudomorphs after an olivine 

 poor in iron : these show the usual crystalline form, and are generally 

 not corroded. The crystals often occur in groups (PI. VII, figs. 2 

 & 3). The original olivine is now completely replaced by pale green, 

 weakly-polarizing serpentine, calcite, and other secondary pro- 

 ducts. The serpentine often shows beautifully the characteristic 

 mesh-structure, and is sometimes exquisitely spherulitic. The 

 ;groundmass commonly shows a confused aggregate of lath-shaped 

 felspars, iron-ores, and apatite, with serpentine, brown mica, calcite, 

 •epidote, titanite, and other secondary products. In the freshest ex- 

 amples (M 118, 214, 216, 218, & 268 ; PI. VII, figs. 2 & 3, and text- 

 fig. 8, p. 170) the original constitution can be fairly well made out. 



1 Mem. Oeol. Surv. Gt. Brit, vol ii, pt. i (1848) p. 56. 

 ^ Trans. Woolbope N. F. Chib, 1869, p. 6. 



^ These three analyses in any case refer with practical certainty to three of 

 the four masses M 108, 247, 248, & 249. 



