316 ME. ALFRED HARKER ON THE GABBRO [Aug. 1 894, 



followed that of the granophyre, while that of the gabbro preceded 

 the acid intrusion. It may be remarked that numerous dykes or 

 veins of granophyre similar to the rock of Carrock Fell occur in the 

 gabbro to the south, but I have not found any of these in the area 

 mapped as diabase. It can scarcely be doubted, however, that the 

 three rocks belong to the same general period of igneous activity, 

 and with them we may include in this statement the numerous 

 basic dykes and veins, of which Mr. Groom's rock is one. These 

 dykes were injected after all the larger igneous masses, for they are 

 found cutting gabbro, granophyre, and diabase alike. The geolo- 

 gical age of this complex of igneous rocks is a question which may 

 be deferred for the present. 



The greisen of Grainsgill is probably quite distinct from the pre- 

 ceding rocks. It must be referred, with the more normal type of 

 the Skiddaw granite, to a late phase of the great post-Silurian 

 disturbances. It is intrusive in Skiddaw Slates, and produces in 

 them an extraordinary degree of metamorphism. 



In the present communication the gabbro alone will be treated in 

 detail, the granophyre, the dykes of Carrock Pell, and the greisen of 

 Grainsgill being reserved for further treatment. 



2. MlNERALOGICAL CHARACTERS OF THE GABBRO. 



Before describing the remarkable variations of the gabbro in 

 different localities it will be convenient to mention the minerals 

 which compose the rocks. A triclinic felspar and a monoclinic 

 pyroxene are essential to all the varieties, though their relative 

 proportions may vary considerably. Quartz and iron ores become 

 prominent constituents in the more acid and the more basic types 

 respectively. Besides these, there are accessory and exceptional 

 minerals and those of secondary origin. 



The felspar occurs for the most part in idiomorphic crystals, 

 which, in fresh specimens, are quite clear. They always show albite- 

 lamellation, and usually Carlsbad twinning in addition, while 

 pericline-lamellse occasionally come in somewhat capriciously in 

 portions of the crystals. The albite-twinning is sometimes 

 inconstant or interrupted, but there is nothing to prove decisively 

 that it is in general of secondary origin, although secondary lamella- 

 tions, both albite aud pericline, are found locally ; see PI. XVII. 

 fig. 2. Sections perpendicular to the lamellae give extinction-angles 

 up to about 32°, indicating a basic variety of labradorite. The specific 

 gravity is a little under 2*69. It is a point of some theoretical 

 importance that the same variety of felspar is found in specimens 

 of the gabbro differing widely in chemical composition. Only in 

 some of the most highly basic rocks does the felspar sometimes show 

 larger extinction-angles, indicating an approach to anorthite. On 

 the other hand, in some of the more acid quartz-bearing rocks the 

 border of a crystal gives a very slightly lower extinction-angle 

 than the main portion, indicating that the outer layers are a 



