458 Alexander Scott — The Crawfordjohn Essexite 



not abundant, also occurs in squat crystals, though sometimes it 

 appears to border the plagioclase. The nephelite is fairly fresh and 

 seldom idiomorphic, being usually found in irregular aggregates. The 

 analcite is always interstitial and very fresh, and is distinguishable 

 by its high relief and absence of double refraction. There are also 

 numerous small eumorphic prisms of apatite. The iron-ore is 

 apparently a titanomagnetite, and is generally surrounded by a narrow 

 rim of deep-brown biotite. 1 



The olivine is invariably the first mineral to crystallize after the 

 accessory apatite and orthite(?) and the felspar the second, since 

 they both occur enclosed in the augite. In some cases, the 

 felspar has begun to solidify while the olivine crystals were still 

 growing. The last-formed minerals are nephelite and analcite, the 

 latter of which has not only filled the interspaces but has also, while 

 still liquid, corroded the already formed phenocrysts with the 

 production of biotite. The formation of the latter mineral is therefore 

 to be ascribed to the action of the water-rich magmatic residuum on 

 the already formed olivine and magnetite. 



Although the porphyritic rock is fairly uniform, local differences 

 in the relative amounts and grain-size of the various minerals are 

 common. Column 1, table i, gives the mineralogical proportions, as 

 determined by the Rosiwal method, of a rock from one of the 

 exposures on the hillside, while column 2 gives those of a rock from 

 the large quarry. The ratio of augite to olivine varies considerably, 

 the variation being quite irregular so far as position in the intrusion 

 is concerned. While one specimen from the large quarry is very 

 poor in analcite and nephelite, the total amount of these minerals 

 being about 3 per cent, others, within a short distance, contain as 



100-0 100-0 100-0 100-0 100-0 



1. Essexite (normal type), Craighead. 



2. Essexite (augite-rich type), Craighead. 



3. Essexite, Carclout, Patna. 4 



4. Theralite, Lugar. 5 



5. Kylite (less femic type), Craigmark. 6 



1 Cf. R. Campbell & A. G. Stenhouse, Trans. Edin. Geol. Soc, ix, p. 126, 

 1907. 



2 Includes some turbid unidentified matter. 



3 Includes a little hornblende. 



4 Cf. G. W. Tyrrell, Geol. Mag. [5], IX, p. 121, 1912. The writer is 

 indebted to Mr. Tyrrell for the loan of slides of this rock. 



5 Ibid., p. 77. 



6 Ibid., p. 123. 



