472 • PKOF. W. S. BOFLTON OS THE [Nov. I904, 



slope, and just outside the wood (573, 513), are dark, fine-grained, 

 basic-looking rocks of much the same type as No. 525, which, in 

 my first examination of the hill, were mapped as intrusive basalt. 

 Microscopically the felspars are milky-white by reflected light, in 

 places blotched with haematite, while the matrix is of a pale 

 yellowish-green. Much secondary ilmenite with leucoxene ; the 

 pale, much-cracked augite of the andesite-lava ; and a hyalopilitic 

 groundmass, with much of the glass converted into green palagonite, 

 are also seen. Circular vesicles are common, filled with concentric 

 zones of a green substance exhibiting well-marked spherulitic 

 structure, a colourless, brightly-polarizing substance, and calcite. 

 These rocks are associated with hard ballefTintas, as appears to be 

 the case generally. The close proximity of the newer basic group 

 probably accounts for the large quantity of secondary iron-ore 

 present, and the consequently more basic appearance of the rock. 



The silica-percentage of Xo. 51 6 c, a typical specimen of the augite- 

 andesite,is 50*67; while the specific gravity of five different specimens 

 from various points on the hill varied from 2'76 to 2*83, giving 

 an average of 2*80. The rock is thus practically basic ; but, from 

 the comparative abundance of felspar (probably oligoclase) and the 

 absence of olivine, it is perhaps more convenient to style it a 

 basic augite-andesite, or andesitic basalt. 



(d) Summary of the Andesite-Grroup. — A marked feature 

 of the Andesite-Group just described is the preponderance of tufts, 

 generally glassy, but sometimes made up almost entirely of broken 

 crystals of oligoclase or andesine. These tuffs are the fragmental 

 representatives of a basic augite-andesite lava, which in places is 

 interleaved with the tuffs. Erom the blade-like character of some 

 of these masses of andesite, and, in places, their tendency to an 

 ophitic structure, it would be unwise to ignore the possibilitv of 

 the intrusion of some of them into the tuffs. But the evidence, 

 both penological and in the field, and especially the occurrence of 

 lapilli of similar andesite in the associated tuffs, seems to point 

 to their bedded origin ; and, in any case, there can be little doubt 

 that both tuffs and andesites belong to the same petrological series, 

 and are of the same general age. 



The quantity of palagonitized glass in these tuffs and lavas is 

 remarkable, and equally so the comparative freshness of the 

 palagonite, considering the great antiquity of the rocks. 



The substance, palagonite, is not uncommon in the older glassy 

 volcanic rocks of Britain and elsewhere, both in basic tuffs, and as 

 an alteration-product of the glassy residue of basic lavas. Thus 

 Prof. Cole has described and figured it in the andesite-tuff of Snead 

 near Bishop's Castle, as well as in the associated andesite-lavas • ; 

 and palagonite-tuffs in the Carboniferous rocks of the Forth Basin, 



1 ' On some Additional Occurrences of Taehylyte ' Quart. Journ. Geol. Soc. 

 toI. xliv (1688) pp. 305-306 & pi. xi, fig. 5. 



