478 PKOF. W. S. BOTJLTON ON THE [Nov. I904, 



hill. While it may be impossible to disprove its intrusive origin 

 (for the banded and pyromeridal structures do not necessarily 

 negative its intrusion), it seems more in accordance with the 

 facts l to consider the rhyolite as an outpouring of lava, and to regard 

 its junction with the andesite-tuffs as a break in the history of the 

 volcanic activity represented by the Pontesford rocks. 2 



After leaving the Northern Rhyolite, the whole of the tuffs and 

 lavas, including the acid breccias and rhyolite at the south-eastern 

 end, form a continuous bedded series, despite the great difference 

 in the average silica-percentage of the Andesite-Group and the 

 Rhyolite-Breccias near the South-Eastern Rhyolite. Commencing 

 with a silica-percentage of nearly 60, these andesite-tuffs (together 

 with their associated lavas) become practically basic, with a little 

 over 50 per cent., and end with tuff and lava of a pronounced acid 

 type, with a percentage of about 75. 



Thus, even if the Northern Rhyolite should be regarded as 

 intrusive (and to determine this finally, evidence from adjacent 

 Uriconian areas ma}- have to be considered), the South-Eastern 

 Rhyolite must be regarded as bedded. 



It is impossible to point definitely to the source of these bedded 

 volcanic rocks, but from the thinning of the tuffs towards the north- 

 east, and the diminution of the size of their lapilli, together with 

 their more gritty and washed appearance, when followed in this 

 direction, it might be inferred that they had their origin in some 

 vent or vents to the west of the present site of Pontesford Hill. 



(7) The Intrusive Basic Rocks. 



The basaltic rocks that make up the higher ground of the hill 

 vary considerably in colour and texture from point to point. 

 Typically, the rock is a dark or purplish-red, coarsely-amygdaloidal 

 dolerite or diabase, well shown along the eastern side, where it has 

 weathered into bare, bold cliffs. But in other places it is iron- 

 grey, very hard, fine-grained, and compact (60), or again somewhat 

 coarsely crystalline, and showing to the naked eye a marked 

 ophitic structure (35, 514, etc.). In places along the Camp at the 

 top of the hill, and elsewhere, the rock has an intense red colour, 

 due to the large amount of haematite contained in it. Specimens 

 may be collected showing a breccia-like appearance, the angular 

 fragments differing slightly in texture and colour from the sur- 

 rounding material, as if a partly-consolidated mass had been broken 

 up by subsequent intrusion. In other places the rock exhibits 

 a spheroidal structure (424), the spheroids measuring sometimes a 

 foot across. 



1 If the ' inlier ' of rhyolite-rock (559), described on pp. 455-56, be a true 

 tuff, the evidence for the extrusive origin of the rhyolite would seem fairly 

 eoinplete. 



2 For a description of an ancient, bedded, volcanic group, with sudden and 

 marked changes in chemical composition, see Sir Archibald Geikie's 'Ancient 

 Volcanoes of Grreat Britain ' vol. i (1897) pp. 145 et seqq., and Quart. Jo urn. 

 Geol. Soc. vol. xxxix (1883) pp. 300 ct seqq. 



