Vol. 60.] IGNEOUS ROCKS OF PONTESFORD HILL. 455 



are inverted, can only be definitely determined after the entire 

 Uriconian and Longmyndian groups of the Longmynd, etc. have 

 been mapped in detail. 



The bedded rocks of the hill, including the rhyolites, andesites, 

 and tuffs, thus have a total thickness of about 3200 feet. 



All the higher portions of the hill are made up of olivine-dolerite 

 and basalt, that has forced its way up. mainly along two planes, 

 overspreading the bedded rocks, and forming a laccolite-like mass, 

 now separated into two parts by the north-easterly and south- 

 westerly cross-gulley. 



III. Detailed Description of the Rocks. 



(1) The Northern Rhyolite. 



This is typically a hard, massive, highly-siliceous rhyolite of a 

 pale-pink or purple colour, and showing many of the characters of 

 Uriconian rhj-olites, which have been so admirably described b) T 

 Airport. 1 Throughout the rock there is much yellow epidote, 

 green chlorite or viridite, together with calcite and secondary 

 quartz and chalcedony, either filling vesicles, or in veins traversing 

 the rock in all directions. 



On the north-western flank of the hill, and in the upper part of 

 the mass, as far east as the larger of the two faults marked on the 

 map, the rock is in general well banded, with very small elongated 

 vesicles filled with quartz, the lines of flow runniug round them (6). 2 

 The gnarled fluxion-banding is well shown at (432), where the 

 strike of the bands is north-east and south-west. East of the 

 smaller of the two faults, however, the banding runs nearly due 

 east and west, as may be well seen at (42) and (43). The dip 

 of these bands is 30° or less at the extreme northern end, but it 

 increases southward, so that at (42) it is 40°. 



On the north-eastern side of the hill, nearly halfway up the 

 steep slope, there is an exposure of the rhyolite some 60 yards 

 wide, separated from the main mass by about 150 yards of dolerite. 

 It is hard, dense, and pink in colour, and has a brecciated look, as if 

 it might be a tuff. At the junction with the dolerite, the rhyolite 

 is considerably discoloured, and shows clear marks of having been 

 affected by the basic intrusion. Microscopically, this rock (559) 

 has a very breccia-like appearance, made up of small equal-sized 

 grains measuring about 0*001 inch in diameter, but without distinct 

 outlines. Between crossed nicols the whole mass is seen to be micro- 

 crystalline, with here and there angular and broken crystals of 

 felspar and quartz. The rock is clearly either a very fine-grained 

 rhyolite -tuff, or a rhyolite which has become finely brecciated 



1 'On certain Ancient & Devitrified Pitchstones from the Lower Silurian 

 District of Shropshire' Quart. Journ. Geol. Soc. vol. xxxiii (1877) p. 44!'. 



2 Throughout this paper the numbers in parentheses refer to rock-specimens 

 and sections in the author's Pontesford collection, the localities of the more 

 important of these being indicated on the map (PI. XXXVIII). 



