454 PROF. W. S. BOULTOX OX THE [NoV. I904, 



south-west. The more easterly strike becomes more pronounced as 

 we pass from south-west to north-east, and therefore, as already 

 pointed out. it is nearly east and west in the rhyolite at the 

 extreme north-eastern end of the hill. 



A pronounced jointing, affecting both the older bedded series and 

 the newer intrusive dolerite and basalt, runs from west-north-west 

 to east-south-east, that is, at right angles to the average strike. 



The general sequence of the rock-groups in the hill, beginning 

 from below at the northern end, may be summarized as follows (see 

 sections, PI. XXXIX) :— 



A. Bedded Rocks. 



(1) The Xorthern Rhyolite. — A pale-pink and purple 

 rhyolite with much epidote, chlorite, and secondary quartz, showing 

 vesicular, spherulitic, pyromeridal, and banded structures. Thick- 

 ness about 1000 feet. 



(2) The Andesite- Group. — This is made up of felsitic-looking 

 gritty tuffs, pink and green in colour, passing up into and inter- 

 bedded with andesitic glassy (palagonite) and crystal-tuffs, halle- 

 flintas, and lavas. Thickness about 1800 feet. 



(3) Rhyolite-Breccias (glassy and crystal- tuffs) and grits. 

 Thickness about 150 feet. 



(4) The South-Eastern Rhyolite. — Dark-red or purple 

 in colour, coarsely vesicular, often with bright-green and white 

 amygdules, and well-banded. Thickness about 250 feet. 



B. Intrusive Olivine-Dolerite and Basalt, making up the 



higher portions of the hill. 



At the extreme north-western end the rhyolite is dipping at 

 about 30 : south-eastward, but the banded and flaggy rhyolite a 

 little higher up dips 40° southward. The average dip of the 

 andesite-tuffs and lavas is from 70° to 80°, while the South-Eastern 

 Rhyolite and its associated breccias and grits dip at about 85° west- 

 north-westward, a dip opposite to that of the rest of the bedded 

 rocks. 



This arrangement of the beds, together with the general similarity 

 in composition of the acid rocks at the northern and southern ends 

 of the hill respectively, might suggest that the acid rocks of the 

 north are on the same horizon as those on the south ; and that we 

 are dealing, either with a steep (and probably faulted) syncline, or 

 with the anticlinal limb of a fan-fold. But, as "will appear when 

 these rocks are dealt with in detail, important differences exist 

 between the northern and southern rhyolite-rocks, and we must 

 regard the whole of the bedded tuffs and lavas, with the possible 

 exception of the Xorthern Rhyolite (see pp. 477 &47S), as a regular 

 upward succession. But whether the Xorthern Rhyolite is really the 

 stratigraphical base of the series, as would appear probable from 

 the evidence within the limits of Pontesford alone, or whether all 

 the bedded rocks, except the South-Eastern Rhyolite and breccias, 



