0E THE MALVEE.IT HILLS. 



499 



white matter in the section, which, as seen by substage illumination, 

 is indicated by the darker parts in PL XX. fig. 9. This is probably 

 leucoxene, and it is generally associated with an opaque black 

 mineral, which may, consequently, be regarded rather as ilmenite 

 than as magnetite. The rock is a basalt. 



No. 20. At the back of the Cave. Herefordshire Beacon, overlooking 

 Castle Morion Common. — A compact pinkish-brown to pale liver- 

 brown rock of felsitic appearance and not unlike some porcellanites, 

 with splintery to small wavy, almost con choidal fracture, harder than 

 steel, harder, at all events in part, than quartz, since the brilliant 

 pyramidal face of a quartz-crystal was distinctly scratched by sharp 

 corners of the specimen. 



Under the microscope the rock is seen to be filled with minute, 

 sometimes irregularly-shaped, but generally nearly spherical granules 

 with apparently a somewhat high refractive index. 



The aspect of the section between crossed nicols is that of a felsite, 

 and it appears that the feeble light transmitted under these conditions 

 emanates from portions of the otherwise seemingly structureless parts 

 of the ground-mass, since, as a rule, the minute granules are appa- 

 rently isotropic. It is, however, difficult to speak positively as to 

 their absolute isotropy. Here and there crystals ranging from yi-g- 

 inch to smaller dimensions may be seen either isolated or in groups. 

 They are colourless and prismatic in habit. They are traversed 

 transversely to the axis of the prism by lines which may represent 

 a rather irregular cleavage. Between crossed nicols they undergo 

 parallel or straight extinction, and may belong to the rhombic 

 system. I would very doubtfully refer them to topaz, a supposition 

 which is strengthened by the analysis (xlii.) of the Rev. J. H. 

 Timins * of a rock from the same locality, in contact with felstone, in 

 which he detected one per cent, of hydrofluoric acid. An interesting 

 point connected with this rock is an obscure perlitic structure. In 

 the first section examined it was not sufficiently well marked to 

 enable me to form a definite conclusion ; but since, in another 

 section off the same specimen, a similar but better-marked structure 

 (PI. XXI. fig. 7) has been observed, 1 no longer hesitate to describe 

 the rock as a devitrified obsidian with perlitic structure. It may be 

 the rock analyzed by Mr. Timins, No. xliii., described by him as 

 felstone " of a pink colour," or No. xlv. " Porcellanite, north-east of 

 the cave " f. It is probably the latter, but both analyses are here 

 transcribed : — 





Silica. 



Alumina. 



Oxide of 

 Iron. 



Lime. 



Magnesia . 



Loss on 

 Ignition. 



Alkalies 

 and Loss. 



xliii 



xlv 



77- 33 



78- 92 



12-3 

 8-18 



1-33 

 408 



5-39 

 505 



0-91 

 048 



1-45 

 1-19 



1- 29 



2- 10 





* " On the Chemical Geology of the Malvern Hills," Q. J. Gr. S. vol. xxiii. 

 p. 360. 



t Op. cit. p. 360. 



