636 MR. H. H. ARNOLD-BEMROSE (W THE MICROSCOPICAL [Nov. 1 894, 



magnetite. The olivine is entirely altered to cloudy calcite and 

 serpentine. The augite is in small irregular prisms and in porphy- 

 ritic crystals, and generally unaltered. The latter often contain 

 portions of glassy material, and are sometimes cracked ; groups of 

 two or four individuals and twins are frequent. The felspar, 

 unaltered, occurs in porphyritic crystals which often possess zonal 

 structure. The lath-shaped sections vary in size. Symmetrical 

 extinctions of twins on the albite plan give angles of _«>, .7-5' ^-5- 

 The first may be referred to the anorthite group, and the two last 

 perhaps to the labrador- anorthite group. Many of the smaller ones 

 extinguish nearly parallel to their length. The groundmass is black 

 and nearly opaque, and contains magnetite in skeleton-crystals. 



Another specimen (sp. gr. 2-59) consists of five or six pieces of 

 the black rock about | inch in diameter ; the space between them 

 is filled with small fragments of a light-green rock and very little 

 calcite. Three sections were cut from this specimen. The first 

 consists mainly of one of the larger pieces of black rock. The 

 olivine is altered to calcite with serpentine along the cracks ; the 

 largest crystal measures -85 x '5 millim., and they are all well 

 bounded. The augite is scarce, occurring in small grains, prisms, and 

 porphyritic crystals. The felspars vary from 1 millim. in length to 

 less than -04 millim. The ends are sometimes jagged, and sometimes 

 bounded by crystalline faces : portions of the grouudmass are included 

 in these felspars. Symmetrical extinctions of a twin give the 

 angles -, • They are often arranged in groups, and sometimes two 

 are parallel and nearly touching one another, or touch through 

 part of their length as if they had been pushed up to one another. 

 This has been noticed in lapilli by A. Penck, 1 and their juxtaposition 

 is attributed by him to the sudden cooling of the glassy base. Some 

 of the small crystals are arranged in the form of a cross. The ground- 

 mass is a dense black in ordinary light, with the ends of felspars 

 terminating in fine splinters merged in it. The black mass contains 

 a few hair-like microlites, '001 millim. in breadth, which extinguish 

 parallel with their length. In another portion the groundmass is 

 lighter, and consists of a felt of the microlites. Irregularly-shaped 

 and elongated vesicles are filled with calcite. The boundary of the 

 piece towards the inner part of the slide is irregular, and may be 

 compared to the forms of bays and promontories on a map. The 

 felspars near the border are always wholly contained in it and do 

 not project from it, whilst on the edge of the slide, where it is broken 

 by grinding, the crystals are broken across. These appearances 

 denote, I think, that — before grinding — the piece was a complete 

 individual. It is bordered by calcite, in which are smaller portions 

 of the rock with black groundmass, also a few small felspar- 

 crystals and fragments of augite, and small pieces of a very light 

 dirty-brown rock containing felspar-crystals in a glassy base ; see 

 PI. XXV. fig. 4. The second section from the same specimen 



1 ' Studien iiber lockere vulkanische Auswiirflinge,' Zeitschr. deutsch. geol. 

 Gesellsch. vol. xxx. (1878) p. 97. 



