190 



dull green matrix. It is much altered and may be termed a diabase- 

 porphyrite. Corstorphine hill.tt) three miles west of Edinburgh, is formed 

 of a mass of basic igneous rock, intrusive in the Calciferous Sandstone 

 series. The rock was originally a holocrystalline ophitic olivine-dolerite. 

 It is now composed of very large irregular masses of augite which play the 

 role of interstitial matter, greyish pseudomorphs after lath-shaped 

 felspars, crystalline plates and irregular masses of ilmenite, often 

 more or less changed to leucoxene, green aggregates (viridite or ser- 

 pentine) occasionally showing the form of olivine, epidote, calcite, quartz 

 and zeolites. Good specimens of prehnite and pectolite may be obtained 

 in veins and cavities in the rock. The felspar of this rock has 

 been profoundly altered and rarely gives the distinct reaction of 

 this substance under the microscope. The augite on the other hand is 

 remarkably fresh. 



The rock of Dalmahoy Hill has been described by Mr. ALLPORT and by 

 the author of Cole's Microscopical Studies/' 2 * It forms an intrusive sheet in 

 Carboniferous shales and sandstones. Mr. ALLPORT describes it as a black 

 dolerite of distinctly crystalline texture composed of plagioclase, augite 

 and magnetite, with yellowish green pseudomorphs after olivine. The 

 felspar is quite unaltered and exhibits coloured bands in polarized light. 

 Apatite occurs sparingly, in long hexagonal prisms. The spaces between 

 the larger constituents are filled with a felsitic and cryptocrystalline mass 

 instead of the usual glass ; in one section this substance also occurs as a 

 small vein and is crowded with minute hexagonal plates of specular iron. 

 The felspar of this rock gives sections which are broad in proportion to their 

 length. The augite occurs in good sized grains and granular aggregates and 

 is not interfered with in any conspicuous manner by the felspars ; in other 

 words the rock is not markedly ophitic. One of the most interesting con- 

 stituents is the interstitial matter which is present in considerable quantity. 

 It is rendered brown by indistinct granular matter, and contains microlites 

 and skeleton crystals of felspar. The forms of the skeleton crystals remind 

 one of those represented in Fig. 7, page 15. Olivine is certainly rare in this 

 rock, but seems to be occasionally represented by pseudomorphs. 



At Ratho, nine or ten miles west of Edinburgh, occurs a mass of coarse 

 grained dolerite of somewhat variable aspect. The rock is composed 

 essentially of plagioclase, augite and magnetite. A coarse grained variety 

 from the quarry by the canal contains long, bladed augite crystals with 

 traces of diallagic striation parallel to the basal plane. Micropegmatite is 

 present and plays the role of interstitial matter between the felspars. A 

 specimen from Norton quarry contains, in addition to the above constituents, 

 a rhombic pyroxene which, from its pleochroism, may be identified as 

 hypersthene. In the character of the augite, as well as in the presence of 



(1) VOGELSANG describes and figures a basalt from Podlie Crag near North Berwick, 

 (Die Krystalliten, p. 119 and Fig. 1. Plate XIII.). It contains crystals of augite, felspar and 

 probably psuedomorphs after olivine in a ground -mass which is remarkable on account of the 

 presence of numerous skeleton crystals of magnetite (the rows of grains are grouped in straight 

 lines which are often related to each other as the axes of cubic crystals) and augite. 



(2) November, 1882, No. 26. 



