201 



dorite which usually measure from 1 to 5 mm. across and occur in such 

 numbers that three or four may generally be recognized on a square inch of 

 surface. The ground-mass in which the porphyritic crystals lie embedded 

 may be distinctly crystalline, finely crystalline, or compact according to 

 the thickness of the dyke and the position from which the specimen is 

 obtained. At the chilled margin the ground-mass is compact, but the 

 porphyritic crystals are still recognizable. The constituents of the 

 ground -mass are lath-shaped plagioclase, granules and crystals of augite, 

 magnetite or ilmenite and interstitial matter. Crystals of augite are 

 comparatively rare. When they do occur they are considerably larger than 

 the grains and show the common forms. The interstitial matter is usually 

 brown in colour in consequence of the presence of minute indistinct grains, 

 tibres and flecks. In only one instance was an isotropic glass observed, and 

 then the glass was seen to contain globulites and longulites similar to those 

 of the Eskdale dyke. The common, brown interstitial matter is crypto- 

 crystalline and frequently contains microlites and skeleton crystals of 

 felspar (see Fig. 7., p. 15). The rock possesses a specific gravity of 277 and a 

 silica-percentage varying from 57 to 59. It is therefore an andesitic 

 dolerite (augite-andesite). In addition to the constituents mentioned 

 above this rock usually contains quartz, which may be in part original. 

 At Bol am in Durham this dyke expands laterally into a small 

 sill or laccolite from the surface of which the Carboniferous strata 

 have been removed by denudation. Here and also at Cockfield it has 

 been locally altered into a light-coloured substance from which the lime, 

 magnesia, and iron have been almost wholly and the alkalies partially 

 removed. Incipient stages of alteration are indicated by the development 

 of carbonates, and as a consequence of this many portions of the rock 

 effervesce freely when treated with acid. This is a feature which may be 

 commonly observed in basic rocks of all ages. The micro-structure of 

 the ground-mass of this rock, near the margin where it has been cooled 

 rapidly, offers a marked contrast to that of the central portions of the 

 dyke. It is a veritable " mikrolithen filz," that is, a closely matted 

 aggregate of minute felspar microlites. In this respect it differs from the 

 rocks of the basalt-family, and resembles the typical andesites. 



The Acklington Dyke, like that of Cleveland, may be followed at 

 intervals for a very considerable distance. It is seen on the sea-coast at 

 Bondicar, Northumberland, and may be traced from this locality into the 

 Cheviot district. It is probably represented also in the south of Scotland 

 At Acklington it is about 30 feet wide. The rock is distinctly crystalline 

 and of a dark bluish grey colour, which rapidly changes to black on 

 exposure. In mineralogical composition it differs from the Cleveland dyke 

 merely in the absence of any conspicuous porphyritic felspars. In com- 

 position and texture it is constant throughout the entire length examined. 

 Its specific gravity is about 2'81 ; that is, somewhat higher than the Cleve- 

 land Dyke. The rock is an andesitic dolerite. Although the dyke is 

 nowhere seen in contact with any younger rocks than the Coal-measures 

 it may be referred with great confidence to the Tertiary period. 



