Vol. 60.] 



IGNEOUS ROCKS OF PONTESFORD HILL. 



479 



The amygdaloid al type shows vesicles generally elongated in a 

 direction parallel to the strike of the bedded tuffs, sometimes 2 

 or more inches long, and filled with calcite and other secondary 

 minerals, which, however, have in some cases been dissolved out, 

 giving to the rock a very vesicular, slaggy appearance. 



In a small quarry at the extreme south-western end of the hill 

 (431), 1 very fine specimens of the amygdaloidal rock may be seen. 

 The vesicles, up to 2 inches across, which are here quite round, 

 have been filled with small spherulitic growths of red iron-oxide 

 and chalcedony with pronounced concentric rings, the clearer 

 siliceous portions showing a dark cross in polarized light. These 

 bodies generally line the wall of the vesicle, while the inter- 

 vening spaces have been filled mostly with calcite, but also 

 with chalcedony, chlorite, and, in some cases, further spherulitic 

 aggregates of iron-oxide. The rock is here much veined, showing 

 slickensides, and calcite has been deposited in large quantities. 

 Microscopic sections of the rock often show the felspars orien- 

 tated in the direction 

 Fig. 5. — Roughly -parallel wavy ridges on a of strike of the an- 

 iveatliered surface of basalt. (Natural desite-tuffs, and at 

 size.) one place in the 



basalt of the Camp 

 (53), curious roughly- 

 parallel and wavy 

 lines, about one- 

 eighth of an inch 

 apart, are visible on 

 the weathered sur- 

 face, and have the 

 same direction. They stand out as thin ribs, as if made of harder 

 material than the rest of the rock (fig. 5). 2 



Prof. Bonney has described a specimen from the Camp. He 

 says : — 



' The grounduiass is full of elongated microliths of felspar with a slightly- 

 parallel grouping, generally plagioclase, but possibly in one or two cases ortho- 

 clase, with dark granules, probably in many cases haematite, and numerous 

 grains (generally rather irregular in outline) of augite. One of more definite 

 form is a compound crystal, about 0*02 inch in diameter. The rock is a basalt, 

 and more resembles that of a flow than of a dyke.' 3 



It may be added that the rock is a type of the finely-granular 

 dolerite or basalt, and that very little of the original material of 

 the felspars or augite remains, although the outlines of the crystals 

 are perfectly preserved. Silica-percentage = 47*62 ; specific gravity 



= 2-84. 



1 This is the opening referred to by Murchisou : see p. 451. 



2 A thin slice, taken across a selected specimen of the rock, did not reveal 

 any difference in structure or composition such as might account for these 

 curious ribs. The same structure, but on a larger scale, is to be seen in some 

 of the igneous rocks of Llanvawr, in the Ordovician of the Corndon district. 



3 Quart. Journ. Geol. Soc. vol. xxxviii (1882) p. 124. 



