468 



PKOF. W. S. BOULTON OX THE 



[Xov. 1904, 



crystal-fragments, and splinters of yellow palagonite enclosing 

 felspar-needles (PI. XLIII, fig. 3). 



These rocks can be easily followed cropping out along the south- 

 western face of the gulley, but higher up the slope towards the 

 wood at the top of this part of the hill, the beds are found to be 

 harder, more massive and fine-grained, less flaggy and more gritty, 

 until at the top near the wood (512) the rock is a very hard 

 massive grit, dark green with pink grains, having a specific gravity 

 of 2*79, and dipping at 80°. Microscopically, it resembles No. 205, 

 but is not so coarse, and contains many grains of ilmenite altering 

 to leucoxene, a constituent which probably accounts for its rather 

 high density. 



A band of these hard grits, 70 yards thick, can be traced all 

 down the north-eastern slope of the gulley, dipping about halfway 

 down at 60°. They are followed at once on the south side by 

 beautifully-laminated green and yellow hiilleflintas, with inter- 

 bedded andesite-lavas ; while on the other, or north-west, side they 

 are covered by the newer basic rocks. 



Some of the best and most accessible spots for observing the 

 typical andesite-rocks are along the foot of the western slope of 

 the hill, south of the gulley, in the little gardens behind the cottages 

 that occur at intervals along the road. Thus at (209) the following 

 section is seen : — 



Fig. 4. — Section under the fence, western flank of 

 Pontesford Hill [209]. 



[Length of section = about 30 feet ; dip = So .] 



a— Finely-laminated green hallefiintas, 8 feet thick, striking obliquely with 

 the fence and dipping at 85°, containing a red and white siliceous band 

 12 inches thick (b), which, under the microscope (530), appears to be a 

 tuff- band with broken felspar-crystals, but very largely replaced by a 

 mosaic of secondary quartz-crystals. 



c=A coarser, andesite-tutf with circular, oval, and subangular lapilli of ande- 

 site-glass, 2 or more inches across; and interbeclcied with beautifully 

 banded red, blue, and yellow halleflinta. 



The lapilli of andesite-glass occur also in the fine halleflinta, the 

 lines of which flow round them, showing some resemblance to a 

 finely-banded lava with phenocrysts. A lapillus of andesite (531), 

 taken from the finely-banded tuffs, shows microscopically an andesite- 

 glass, pale yellowish-white by reflected light, containing laths of 

 felspar 0*05 inch long, often in radiating groups, much decomposed, 

 but some showing lamellar twinning. Crystals of a pyroxene-like 

 mineral, now replaced by a yellowish-green product, have evidently 



