456 Alexander Scott — The Crawfordjohn Essexite 



The chief exposures are in Craighead Quarry, on the 1,000 ft. 

 contour-line, and in a disused quarry 200 yards to the south-east. 

 A number of isolated exposures are found further up on the hillside. 

 The general trend of the exposures is a line running N.W. and 

 S.E., which is apparently parallel to the northern contact with the 

 sediments. The main rock is well exposed in both quarries, as is 

 a fine-gi'ained marginal facies, but although sediments are also seen 

 the actual contact is usually obscured by drift or debris. The 

 sediments, which are of Ordovician age, consist mainly of grits and 

 shales, folded and lying almost vertical, and have undergone con- 

 siderable metamorphism by the intrusion. The southern contact is 

 nowhere exposed, but the grits occur in situ, in a small drift-covered 

 quarry, 200 yards south of the large quarry. The main mass of the 

 intrusion is a grey compact rock showing numerous phenocrysts of 

 augite. In places, the porphyritic aspect is not so pronounced, and 

 the rock is apparently more even-grained. As the edge of the 

 intrusion is approached, the grain-size diminishes until at the margin 

 the rock is thoroughly aphanitic, none of the minerals being 

 distinguishable in the hand-specimen. 



The Essexites. 



Microscopically, the porphyritic rock is seen to consist of numerous 

 crystals of olivine and euhedral lath-shaped felspars, accompanied by, 

 and occasionally included in, large phenocrysts of augite. Ilmenite 

 and apatite are fairly abundant, and sporadic flakes of biotite also 

 occur. The interstices are filled with nephelite and analcite. 



The olivine occurs in the usual rounded crystals and, as noted by 

 Teall, 1 is generally fresh and distinctly greenish in colour. The 

 crystals are often enclosed by the augite and are euhedral towards 

 the felspar. They contain a number of minute inclusions of a greenish- 

 brown colour. These have no cleavage, are distinctly pleochroic, the 

 colour varying from greenish-yellow to light-brown, and have 

 a maximum extinction of about 35°. The refractive index is high, 

 and the double refraction fairly strong. These characteristics agree 

 with those of orthite, and the presumption that the inclusions are 

 composed of this mineral is strengthened by the appearance of 

 a reddish alteration product and by the fact that a trace of cerium 

 oxide (and possibly thoria as well) was detected during the chemical 

 analysis of the rock. In some of the more altered rocks, the olivine 

 is replaced by pseudomorphs which closely resemble those in the 

 Derbyshire Toadstones described by Bemrose. 2 The alteration com- 

 mences along the cracks, and the pseudomorphs, which have a good 

 cleavage, appear to have the same optical orientation as the original 

 crystals. The pleochroism is very intense, and the colour varies from 

 a blue-green (when the light is vibrating parallel to the cleavage- 

 trace) to a deep red.' The refractive index lies between that of 

 olivine and felspar, the double refraction is fairly high and the axial 

 angle apparently large. Although similar pseudomorphs have been 



1 British Petrography, 1888, p. 197. 



2 H. H. Bemrose, Quart. Journ. Geol. Soc, 1, pp. 603-42, 1894. 



