Vol. 60.] IGNEOUS ROCKS OF POXTLSFOKD HILL. 469 



crystallized out after the felspars, for they often enclose the latter. 

 Irregular, but rounded portions, which macroscopically might be 

 taken for filled-up vesicles, are found to be fragments of previously- 

 consolidated palagonite-tuff, with minute angular bits of palagoni- 

 tized vesicular glass, enclosing needles of felspar, and occasional 

 crystals of augite. These fragments of tuff embedded in the lava 

 have been partly re-fused : for, although of irregular shape, their 

 edges are quite rounded. 



Along the same line (305, 308) are very typical examples of the 

 paiagonite-tuff. In (305) angular lapilli, measuring up to 0*15 inch 

 across, of green and orange-coloured palagonitized glass are present, 

 containing abundant circular vesicles filled with zeolites, together 

 with minute needles of felspar. Many of these lapilli have amarkedly- 

 twisted and slaggy appearance, and some are strongly stained with 

 iron-oxide. The matrix of the rock is a very fine dust, now largely 

 replaced by calcite and other secondar} T miuerals. (PI. XLII, fig 3.) 



Ascending the hill from the gulley towards the Higher Camp on 

 the main summit, the andesite-lavas, coarse and fine andesite- and 

 palagonite-tuffs, with hiilleflintas, are met with in many isolated 

 crags, all along the north-western and western face, extending half- 

 way, in some places two-thirds of the way, up the slope, where they 

 are covered irregularly by the basic rocks (520, 522, 523, 524, 525, 

 etc.). These tuffs are of types already described, but the finer tuffs or 

 halleflintas are hereabouts more common, and are especially well- 

 displayed at intervals along the lower slope, where crags showing 

 beautiful lamination may be seen, as well as quantities of halleflmta- 

 debris brought to the surface by rabbits. 



The following is a complete analysis of palagonite-tuff from 

 Agglomerate Crag, by Dr. C. F. Baker, late of the University of 

 Birmingham : — 



Per cent. 



SiO, 5341 



Al 2 O a 11-52 



Fe,Ol 8-36 



FeO J 3-38 



MnO 1-4S 



CaO 1316 



MgO 263 



K o 0-63 



NajjO 0-71 



Loss at about 110 J Centigrade ... 154 



Loss, extra, at dull-red beat 356 



(Specific gravity = 2-743.) 100-38 



Sir John Murray & the late Prof. Eenard, 1 in comparing an 



lysis of palagonitic matter with that of the anhydrous silicate 



(basic glass), from which the palagonitic substance was derived, say : 



' The transformation which has taken place seems to tend to the formation 

 of a zeolitic substance ; lime and magnesia are eliminated, the protoxide of iron 



1 Challenger Keports : ' Deep-Sea Deposits ' (1801) p. 307. 

 Q. J. G. S. Xo. 240. 2 s 



