Vol. 60.] KtXkofs rocks of pontesfoed hill. 467 



same material, is well seen ; while to the south-east of it is a bright- 

 yellow, hard, and exceeding!}" fine-grained halleflinta, 2 feet thick, 

 which can be traced along the hillside for many yards, thus accu- 

 rately tixiug the strike. 



The following examples show the more typical microscopic 

 characters of these rocks : — 



(Agg. Crag, a.) — Red and green gritty tuff, with fragments 

 measuring, ivp to 0*2 inch across, of decomposed glass, crowded with 

 minute round vesicles, now filled with pale-green doubly-refracting 

 zeolite, together with microlites of felspar showing very low ex- 

 tinction-angles ; crystals of felspar, partly broken, measuring up to 

 O04 inch, with good lamellar twinning ; occasional angular grains 

 of quartz ; twisted pieces of vesicular glass ; lapilli of previously- 

 consolidated glassy tuff, one being made tip of a granular bright-green 

 matrix, full of minute felspar-laths with a parallel arrangement. 



(Agg. Crag, b.) — A good specimen of palagonite-tufF, with lapilli 

 of reddish-brown and bright orauge-yellow palagonite (pale-yellow 

 by incident light) of curvilinear outline, and crowded with minute 

 felspar-laths, and vesicles which are as a rule perfectly circular, but 

 sometimes much elongated. These have usually a ring of clear 

 doubly-refracting zeolite, with a similar material, or. in some cases, 

 a yellow isotropic substance, in the centre. Some amygdules show a 

 black cross with polarized light ; and there is much dirty-white 

 calcite in the matrix. (PI. XLII, fig. 2.) 



(Agg. Crag, c.) — Shows a fragment, - 75 inch across, with fine 

 red and green bauds, embedded in a matrix of pink and green 

 grains. This matrix is a fine crystal-tuff, made up of broken 

 crystals of plagioclase with repeated twinning, pinkish-brown in 

 colour, and set in a fine green dust, while the lapillus consists of 

 alternating bands of purplish-red dust and crystal-fragments. 



(Agg. Crag, d.) — A green tuff, with dull green and pinkish-brown 

 lapilli measuring upwards of 0*75 inch across. These consist of 

 black glass with felspar-microlites ; pale-red, altered glass with 

 many round vesicles filled with a green substance, one pear-shaped 

 fragment of glass 0*06 inch long showing marked perlitic structure. 



Xo. 205 — near Agglomerate Crag — is a coarse, pink-and- green 

 gritty tuff, showing well all the different kinds of lapilli, which 

 measure generally about 0*1 inch across (PI. XLII, fig. 1). An 

 included fragment in the tuff, of a pale yellowish-green, is a piece of 

 decomposed audesite-lava, the matrix being crowded with felspar- 

 needles, milky-white by incident light, and containing vesicles filled 

 with pale yellowish -green, doubly-refracting zeolite with spherulitic 

 structure. 



A buff-yellow finely-laminated halleflinta. 1 near by, shows bands 

 of very fine glassy dust, alternating with coarser bands made up of 



1 The term 'halleflinta ' is here used, as elsewhere in this paper, as a general 

 field-term for a bard, felsitic, fine-grained, laminated rock. In Pontesford 

 Hill all the halleflmtas are fine glassy and crystal-tuffs of andesitic com- 

 position. 



