Vol. 60.] IGNEOUS KOCKS OF PONTESFORD HILL. 473 



and the Pebidian of St. David's (Pembrokeshire), have been figured 

 and described by Sir Archibald Geikie 1 ; while Prof. Zirkel has 

 described tuffs of this nature from Xevada and elsewhere. 2 But 

 the finest palagonite- tuffs are the more recent ones of Sicily, 

 Iceland, the Canary Islands, etc., including those of Palagonia with 

 the type-palagonite of Waltershausen, the characters of which 

 have been summarized by Prof. Penck. 3 Through the kindness of 

 Prof. Judd, I have been able to examine some of these rocks, as 

 well as specimens from Samoa, given to me by Mr. H. T. Waller. 



In the palagonite-tuff from Gaidar (Grand Canary), lapilli of 

 orange, reddish-brown, and yellow palagonite (average measurement 

 = '2 iuch across), contain fresh clear olivine-phenocrysts ; circular 

 vesicles lined or filled with zeolites; and the same zeolite (phillipsite) 

 forms a fibro-radiate. mammillated border round the lapilli, the 

 outside margin of this border having a bright-yellow colour. 4 



In the Samoan rocks the palagonite is yellow, orange, or reddish- 

 brown, with a singular absence of separated iron-oxide, and enclosing 

 microlites of felspar, and phenocrysts of fresh, nearly colourless 

 olivine, together with round or elongated vesicles filled with 

 zeolites. 



The tuff from Samoa contains lapilli, up to half an inch across, 

 of yellow and orange-yellow, faintly-polarizing palagonite, crowded 

 with minute and perfectly-round or much-elongated vesicles, and 

 containing fresh, nearly-colourless phenocrysts of olivine. The 

 vesicles are mostly filled with zeolites, a clear, colourless border of 

 a doubly-refracting substance, and a dark, nearly-opaque centre 

 of minute brown granules, possibly iron-oxide, the whole giving a 

 dusky cross in polarized light. Distinct from these vesicles, and 

 much smaller, minute gas-pores are visible, often filled with pala- 

 gonite ; while others, with a faintly-marked radial and concentric 

 structure, are slightly affected by polarized light, and probably 

 represent globulites, or the variolitic structure on a small scale. 

 In one specimen from Samoa, each fragment of pale yellowish-brown 

 palagonite, crowded with microlites and skeleton-crystals of felspar, 

 is ringed round with a darker border of orange-yellow palagonite, 

 and the vesicles have a border of the same brown material, the 

 centres being filled with colourless zeolite. Except for the presence 

 of olivine in these rocks, and the somewhat fresher, clearer, and 

 almost isotropic character of this palagonite, there is scarcely a 

 detail of structure and appearance that cannot be matched iu the 

 palagonite-tuffs of Pontesford. 



In the volume on the Deep-Sea Deposits of the Challenger 



1 Trans. Eoy. Soc. Edin. vol. xxix (1880) pp. 513-16 ; and ' On the supposed 

 pre-Cambrian Eocks of St. David's ' Quart. Journ. Geol. Soc. vol. xxxix (1883) 

 pp. 295-300. 



2 U.S. Geol. Explor. Fortieth Parallel, vol. vi ' Microscopical Petrography 

 (1876) pp. 272-75 & pi. xii, figs. 3-4. 



3 ' Eeber Palagonit- und •Basalttuffe ' Zeitschr. d. Deutsch. Geol. Gesellsoh. 

 vol. xxxi (1879) pp. 504-77. 



4 Compare Challenger Eeports : 'Deep-Sea Deposits' (1891) pi. xviii. 

 figs. 2 & 3. 



