Vol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES ASTD TUFFS. 639 



Another block (sp. gr. 2*48) contains similar minerals, but the 

 base is often a dense black, sometimes a dark grey, and isotropic. 

 The vesicles are filled with calcite or with the felspar-like mosaic, or 

 both. Some vesicles contain lapilli without crystals, which are altered 

 to a mosaic in the inside, and bordered by a brown material which 

 contains globulites, is almost isotropic, and in reflected light has a 

 dirty porcellanic appearance. At one edge of the slide is a large 

 group of these lapilli in a felspar-mosaic, which is probably an 

 amygdaloid. 



A section from another block (sp. gr. 2*59) consists of a similar 

 rock bounded by a calcite-cemeot, which contains small glassy 

 lapilli without crystals, some of them altered to calcite. These 

 lapilli are probably portions of the ash adhering to the block. 



In another block (sp. gr. 2*70) the felspars sometimes appear to 

 resemble twins, but both parts invariably extinguish together and are 

 separated by a portion of the base containing globulites. The smaller 

 felspars are often arranged in bundles and plume-like forms, 

 radiating from a point so as to form a small sector of a circle. 

 There is very little base, consisting mainly of globulites in glass 

 (isotropic). 



In a thin section of tuff (sp. gr. 2 - 40) the lapilli vary from a 

 colourless, through a yellow to brown or dark-brown glass. A few 

 have a slight action on polarized light. The vesicles are of the 

 same glassy substance, and are often surrounded by a border of 

 globulites or iron oxide, or both. One large lapillus only contains a 

 few felspars in lath-shaped sections altered to calcite. 



Another thin section (sp. gr. 2*44) contains lapilli, very irregular 

 in shape and amygdaloidal. In some cases they are a mass of 

 amygdaloids with narrow walls. There are a few probable pseudo- 

 morphs of olivine, but no felspar. A lapillus has a black base con- 

 taining pale yellow vesicles with a darker yellow border; they have 

 a bright action on polarized light and a radio-fibrous structure. 

 Other lapilli are of dusty dark-brown or yellow glass, and others 

 more like tachylyte, with sometimes a slight action on polarized light. 

 Some lapilli are altered in part to calcite, and in part to a fine 

 felspar-like mosaic. "When entirely altered to calcite, the lapillus is 

 traversed by cracks which are rilled with iron oxide. Calcite 

 forms the cement between the lapilli. 



A third specimen (sp. gr. 2*46) contains lapilli with olivine 

 altered to calcite, and felspar in small lath- shaped sections, crowded 

 together and often in bundles, in a base partly black and partly 

 transparent and isotropic. Other lapilli are of clear and transparent 

 glass, containing strings and rods of globulites or longulites. Some 

 are altered to a felspar-like material which has very feeble action 

 on polarized light. 



Ashover, Outcrop 59. — This occurs in a small inlier of Mountain 

 Limestone which has been brought up by an anticlinal. Exposures 

 are best seen in two cuttings which have been made to lime-kilns 

 on the right of the road from Milltown to Ashover. The rock is less 

 weathered in the cutting nearer Milltown. In a small cave on the 



