474 PROF. W. S. BOULTON ON THE [Nov. 1904, 



Eeports, 1 Sir John Murray & the late Prof. Renard described 

 palagonite-, glassy-, and costal-tuffs, dredged from the bottom of 

 the ocean, which again show characters almost identical with 

 those of the Pontesford rocks. 



It seems probable that the conversion of the basic andesite-glass 

 into palagonite, in the case of the Pontesford rocks, took place 

 soon after their eruption, and that further and later changes in 

 the rocks have affected the crystalline constituents and fine matrix 

 of the tuffs, rather than this palagonitized glass. It has been con- 

 tended by Prof. Penck, Sir John Murray & the late Prof. Eenard, 2 

 and others, that the conversion of basic glass into palagonite is 

 brought about largely by the hydrochemical action of sea-water, 

 whereby changes take place which tend to the formation of zeolites. 

 That the tuffs of Pontesford were deposited in water is abundantly 

 clear, from the fine and regular lamination of some of the tuffs and 

 hallenintas, and the pronounced bedding of some of the volcanic 

 grits. 



(4) Rhyolite-Breccias and Grits associated with the 

 South-Eastern Rhyolite. 



It will be seen from the map (PI. XXXVIII) accompanying this 

 paper that a strip of dolerite at the southern end of the hill inter- 

 rupts the succession of the bedded volcanic group. Andesite-tuffs 

 and lavas can be traced right up to this dolerite on the western 

 side, and the same rocks are met with along the footpath in the 

 adjacent field (535) cropping out through the dolerite, which makes 

 lip nearly all the ground at this extreme southern end. On the 

 eastern side of the dolerite, along the footpath by the side of the 

 fence, which roughly corresponds to the little cross-fault marked on 

 the map, the andesite-rocks are again met with in small and rather 

 obscure outcrops (536, 537). No. 536 is much brecciated, iron- 

 stained, and under the microscope shows a large amount of 

 secondary quartz (it yielded on analysis 84*70 per cent.). If the 

 longitudinal fault through the centre of the hill (see p. 453) runs as 

 far south, it would probably come through this point. 



No. 537 is a pale, siliceous-looking rock, with a distinct banded 

 appearance, the bands running parallel to the general strike of the 

 andesite-group. Under the microscope it is found to be a crystal- 

 tuff, with broken crystals of felspar 0*01 to O07 inch long, showing 

 both simple and lamellar twinning, and set in a finely-banded, 

 dusty matrix, containing a few lapilli of decomposed vesicular 

 glass. 



No. 17 L is a fresher-looking rock, but with much iron-staining, 

 and yellow and green secondary products. It is full of small 

 felspar-laths with a parallel arrangement, extinguishing parallel 

 (or nearly so) to their length, with a few larger crystals showing 



1 Pp. 304-311 & pi. xviii. 



2 Ibid. p. 307. 



