464 PROF. AV. S. JBOULTON ON THE [Nov. I904, 



and some secondary silica. The rock is undoubtedly a tuff, and 

 from the abundance of simply-twinned felspar, and the felsitic 

 look of the lapilli, apparently more acid than the andesite -tuffs 

 higher up. 



No. 554. — A dull -green and red ashy-looking rock, with 

 55-8 per cent, of silica and a specific gravity of 2*694. Micro- 

 scopically, it is clearly a fine ash with lapilli measuring up to 0*05 

 inch across, made up mostly of decomposed glass with skeleton- 

 crystals and microlites of felspar, most with simple twinning, but 

 .some (one O05 inch long) showing lamellar twinning. The rock is 

 rather more basic-looking than No. 566, and the fragments are 

 much stained with iron-oxide. 



No. 551, of pinkish colour, much-jointed, fine-grained, weathering 

 a dull green, is exposed at the back of a ruined cottage, near the 

 road. (Silica-percentage =57*07; specific gravity =2-57.) Under 

 the microscope, it is seen to be a very fine-grained tuff, made up 

 largely of broken crystals of felspar with simple and lamellar 

 twinning (0*001 inch or less), minute particles of reddish-brown 

 glass, with sharp edges and curvilinear outlines, and containing 

 minute vesicles, together with very few quartz-grains. 



All these tuffs and grits clearly belong to the Andesite-Group, 

 for they pass at once without a break into the typical palagonite- 

 tuffs, and indeed are interbedded to some extent with them. From 

 their colour and texture and lower specific gravity, one is tempted 

 to class them with the rhyolite as a group of felsite-tuffs, rather 

 than with the andesites ; but they contain, on an average, only 

 about 5 per cent, more silica than the palagonite-tuffs, and their 

 microscopic characters are practically the same as those of many of 

 the andesite-tuffs higher up the series. It may be here remarked 

 that many of the small pink chips and lapilli in these tuffs, of a 

 pronounced rhyolitic or felsitic appearance in the hand-specimen, 

 generally show under the microscope precisely the same characters 

 as those in the tuffs which, on analysis, prove to be of andesitic 

 composition. The felsitic appearance is doubtless due, in part, to 

 the smallness of the grains allowing of the complete oxidation of 

 the iron to the ferric state. 



(b) Palagonite-Tuf fs, Grits, and Halleflintas. — Behind 

 the cottage at the top of the road leading to Pontesbury (201 on 

 the map, PI. XXXVIII) occurs an interesting exposure, showing 

 the newer basic rocks penetrating the tuffs, and both faulted against 

 the buff-green shivery Shineton Shales, that abut against the hill 

 (see fig. 3, p. 465). Here (201 a, b, c, d, x) we get, for the first 

 time, the palagonite- tuffs of the Andesite-Group. They are dull- 

 green, bluish when fresh, but weathering yellowish-green, and fine- 

 grained with white flecks. 



No. 201 a consists of irregular fragments of yellow and greenish- 

 yellow decomposed glass measuring up to 0*04 inch across, including 

 small round vesicles and minute felspar-microlites, and with curved, 



