Vol. 50.] STRUCTURE OF CARBONIFEROUS BOLERITES AND TUFFS. 637 



contains one piece of the rock with black base ; the psendomorphs 

 of olivine and the augite and felspar contain more of the ground- 

 mass than in the first case. The numerous lapilli are of a light- 

 yellow colour, and so completely isotropic that they are indistin- 

 guishable from the black rock under crossed nicols. Magnetite-dust 

 is scattered sparsely through them, and they contain small augite- 

 grains and prisms and felspar-laths, but no olivine or microlites. 

 Felspar and augite occur singly in the calcite-cement. 



The third section consists of lapilli of the black rock with smaller 

 felspars and augite-prisms, and of yellowish-green and brown lapilli 

 in a cement of crystalline calcite. The latter contain one fresh 

 olivine-crystal and some larger ones, replaced in part by a fibrous 

 brown substance. The augite is in very small prisms and in small 

 porphyritic crystals. The felspars are numerous and vary in size, 

 often having jagged ends. They are frequently grouped, and in some 

 lapilli they are bordered with magnetite. The groundmass is in 

 most cases isotropic, in some cases a slight action on polarized light is 

 observed, and there are a few circular spots which show a more or less 

 regular black cross under crossed nicols. These were probably vesicles 

 originally. Sometimes the groundmass is cracked, and the cracks are 

 filled with magnetite. They do not run through the crystals, except 

 in very few cases ; the same crack is continued on opposite sides of 

 a felspar-crystal, or when it touches the end turns off at an angle. 

 There is no perlitic structure. In some lapilli a certain amount of 

 alteration appears to have taken place, and some have a dirty 

 colour which is probably due to iron oxide. 



Lapilli of another specimen are of a brown colour. In ordinary 

 light they are differentiated into a darker brown portion and into 

 a lighter dusty one : the lighter part often forms what look like 

 cracks in the brown part. In polarized light the whole is iso- 

 tropic. 



Kniveton, Outcrop 54. — This is a small outcrop in an inlier of 

 Mountain Limestone, exposed near the corner of a field a short 

 distance from the footpath which runs from Lea Cottage to Tissington 

 Wood Farm. The exposure is about 6 feet deep and 10 feet in length. 

 The rock is hard, and has a rough bedding along which it breaks 

 more easily than in any other direction. It is evidently a fragmental 

 rock, and does not contain spheroidal blocks such as those of 

 outcrop 56 (see p. 638). 



A large portion of a slide (sp. gr. 2*51) is occupied by one 

 lapillus, which is like the first block described from outcrop 56. It 

 is exactly similar in microscopical structure. In one amygdaloid 

 the felspar-like mosaic consists of separate portions large enough 

 to show a biaxial figure in convergent polarized light. The slide 

 contains several other similar large lapilli, with a more altered base, 

 and the amygdaloids in them contain smaller lapilli. Some of tho 

 smaller lapilli in the remainder of the slide contain no crystals, and 

 are often altered to the felspar-like material. The cement is 

 calcite. 



