148 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



pyramidal) may generally be detected, to a granitoid rock of medium grain, in which 

 the component dull felspar and clear quartz can be readily distinguished by the 

 naked eye. Throughout all the varieties of texture there is a strong tendency to the 

 development of minute irregularly-shaped cavities, which here and there give a carious 

 aspect to the rock. That these cavities, however, are part of the original structure of 

 the rock, and are not due to mere weathering, is shown by the well-terminated crystals 

 of quartz and felspar which project into them. On a small scale, it is the same structure 

 so characteristic of the granite of the Mourne Mountains and of parts of that of Arran. 



Examined under the microscope, a normal specimen of the granophyre of the Western 

 Isles presents a holocrystalline ground-mass, which fills all the interspaces between the 

 crystals of earlier consolidation. This ground-mass consists of an aggregate of clear 

 quartz and turbid orthoclase, arranged in the structure known as micropegmatite. In 

 some parts these two minerals are grouped in alternate parallel fibres, diverging from the 

 surface of the enclosed crystals, which are thus more or less completely surrounded by a 

 radially fibrous mass. In other parts, the felspar forms a kind of network, the meshes of 

 which are filled up with quartz. Through the ground-mass are scattered crystals of clear 

 quartz and dull orthoclase, generally with some ferro-magnesian or other additional 

 constituent, usually somewhat decomposed. In some varieties Dr Hatch has found an 

 abundant brown mica, as in the rock at Camas Malag, Skye. In others, a pyroxene occurs, 

 which he finds in minute greenish grains, sometimes completely inclosed in the quartz. In 

 a third variety the dark constituent is hornblende, the most remarkable example of which 

 is one to be seen at Ishriff, in the Glen More of Mull, where the ferro-magnesian mineral 

 takes the form of long dirty-green needles, conspicuous on a weathered surface of the 

 rock. A fourth variety is distinguished by containing plagioclase in addition to or instead 

 of orthoclase. In the rock of the sheet forming Cnoc Carnach, near Heast, in Skye, Di- 

 ll atch has observed both orthoclase and plagioclase scattered through a fine micro- 

 pegmatitic ground-mass, and in a part of the boss at Ishriff he has found the rock to be 

 composed mainly of plagioclase, in a micropegmatitic ground-mass of quartz and felspar, 

 with a few scattered grains of a pale brown augite and grains of magnetite. A fifth 

 variety is marked by the prominence of the crystals of quartz and felspar of earlier 

 consolidation, and the fineness of grain in the surrounding micropegmatitic ground-mass, 

 whereby a distinct porphyritic structure is developed. Eocks of this kind are macro- 

 scopically like ordinary quartz-porphyries. 



The granophyres occur sparingly as veins or dykes. Conspicuous examples of their 

 assumption of this form are to be seen in the light-grey veins which break through 

 the dark gabbro at the lower end of Loch Coruisk, Skye. More massive and striking are 

 the great dykes that run up the basalt-terraces on the north side of Loch Sligacban. In 

 the form of sheets intruded between the Jurassic strata, or between them and the base of 

 some overlying series now removed, the granophyres (having the general aspect of quartz- 

 porphyries) play an important part in the geology of Strath, in Skye. But it is as bosses 

 that the granophyres attain their largest and most characteristic development, their 



