Vol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 627 



with a well-defined outline, and entirely altered to a network, or in 

 some cases an opaque mass of iron oxide. The felspar is altered, and 

 has only a feeble action on polarized light. It occurs in rhombic 

 sections, one of which has an acute angle of 55°, and in lath-shaped 

 sections which often contain portions of the base. The base is red 

 in reflected, and a dense black in transmitted light, and, when 

 magnified 200 diameters, some portions are seen to have a reddish 

 tinge. The colour is probably due to iron oxide. Some portions 

 of the base are structureless and free from miuerals, and in other 

 places a few hair-like crystallites are present. About half of the 

 section of this lapillus consists of vesicles var}'ing much in size ; 

 the larger ones especially are filled with calcite, which is fresh and 

 shows the usual cleavages. The walls between adjoining vesicles 

 are often very thin. Some vesicles are filled and others fringed 

 with a light yellow substance, which also forms a border to the 

 lapilli. It is grey in polarized light, and has but little action on it. 

 The smaller lapilli are similar, and unbroken across the vesicles, 

 being fully formed individuals, and not fragments of a compact 

 rock. The lapilli are cemented by crystalline calcite. 



A specimen of the more compact rock (sp. gr. 2*70) consists of 

 lapilli varying in size from an inch downwards, of a dark-chocolate 

 or of a green colour, in a cement of calcite. Under the microscope, 

 some lapilli are of a light-green colour, and isotropic, and contain 

 magnetite ; others are similar to those in the specimen previously 

 described. The olivine is altered to a light dirty-yellow material, 

 and the cracks are filled with iron oxide. The outline is often 

 clearly defined by a thiu border of lighter material nearly trans- 

 parent, and having hardly any action on polarized light. The base 

 in some lapilli is a very dark brown, almost black (see PL XXIV. 

 fig. 5), and in others a lighter brown, which, under a magnification 

 of 200 diameters, is resolved into a number of crystallites with 

 parallel extinction ; they have a very feeble action on polarized light ; 

 the remaining base is isotropic. Numerous vesicles are filled with 

 calcite, and ail the vesicles have a very delicate outline. 



Another similar specimen (sp. gr. 2*60) contains groups of a 

 mineral in yellow grains. They are diehroic, with cleavage-cracks 

 parallel to their length. The greatest absorption is parallel to the 

 cracks, and the extinction is always parallel to them. This mineral 

 is probably a rhombic pyroxene. Many of the lapilli are elongated 

 in one direction. 



Two blocks embedded in the tuff were examined microscopically. 

 One of sp. gr. 2- 74 is very amygdaloidal (calcite). Olivine occurs 

 replaced by magnetite or ilmenite. There are few felspars, none of 

 which are fresh ; some of them are altered to calcite. The base con- 

 sists of felspar-microlites and magnetite-grains, or a reddish-coloured 

 substance. The other block (sp. gr. 2-68) is a very fine-grained 

 rock, and much altered. It contains pseudomorphs of serpentine 

 after olivine, felspar-microlites, and magnetite. 



Litton, Outcrop 8. — This is mapped by the Geological Survey from 

 Q. J. G. S. No. 200. 2 x 



