476 PROF. W. S. BOULTON ON THE [Nov. T904, 



The red-and-green grit is followed by a breccia of the type 

 o£ that on the other side of it (538, etc.), but distinctly coarser, 

 with a pinkish matrix containing angular yellow and green 

 splinters measuring 1 inch or more across. Sometimes the rock 

 is bright bluish -green with pink glassy splinters, making up one of 

 the most striking rocks of Pontesford Hill. 



These three bands (the red grit and glassy breccias above and 

 below it) can be followed along the south-eastern flank of the hill, 

 extending nearly up to the Camp, where they abut irregularly 

 against the basalt. In the glassy breccia (545) a vein of barytes 

 about a foot thick, running nearly east and west, has been partly 

 exposed. 



(5) The South-Eastern Rhyolite. 



The bedded rocks of the hill end southward in a rhyolite which 

 skirts it on the south-eastern side, and extends to the eastern 

 boundary-fault. It is a dark purple-red rhyolite, in some places 

 compact, but generally slaggy and coarsely vesicular and amyg- 

 daloidal, the vesicles measuring often 1 inch or more in length, 

 sometimes drawn out into fine tubes, and filled with yellow and 

 green secondary minerals. 



Under the microscope, the vesicular, slaggy and banded structures 

 are very pronounced ; there is much staining with red iron-oxide, and 

 occasionally phenocrysts of felspar are present, generally showing 

 albite-lamellation. Much secondary quartz, yellow epidote, and 

 green chlorite, frequently in spherulitic aggregates, together with 

 radial growths of a colourless, brightly-polarizing, fibrous substance, 

 fill cracks and vesicles. Some of the larger irregular vesicles are 

 partly filled with highly-vesicular and spongy rhyolite, squeezed in 

 while the rock was still plastic ; while, in other cases, sharp, angular 

 portions of the felsitic matrix have been forced in by movement 

 more probably after partial or entire consolidation, as in the case of 

 the more angular fragments of fibrous felsitic matter in the quartz- 

 am ygdaloids of the Northern Rhyolite. 



In some specimens, the rock appears to consist of two magmas 

 that have imperfectly mixed, a darker and more ferruginous one 

 irregularly penetrating a paler variety ; while, in other cases, the 

 bands vary considerably in colour, owing to the irregular distribu- 

 tion of the iron-oxide, so that the rock has a peculiar gnarled and 

 twisted appearance, suggestive of the knotty or grained structures 

 of wood. This gnarled structure is doubtless to be explained by 

 the partial separation of a more basic and ferruginous constituent 

 of the original rhyolite-magma before the extrusion of the lava. 



In one place there is an included fragment, 0*05 inch across, of 

 nearly-black glass with clear vesicles. In a slice (1 Y 1) of one of 

 several of these rocks kindly lent to me by Mr. Parkinson, a well- 

 marked spherulitic structure is visible to the naked eye, the sphe- 

 rulitic bodies measuring 0*1 inch across. The rock was originally 

 the usual highly-vesicular and slaggy type of this South-Eastern 



