42 ME. H. W. MONCKTON ON A PICRITE [Feb. 1 894, 



picrite of Inchcolm, but the latter rock is less altered. 1 I have, 

 however, in my collection specimens which show more complete 

 change and so approximate more closely. After looking through 

 a considerable number of specimens of picrite in my collection I 

 have no hesitation in saying (1) that the Barnton rock is a genuine 

 picrite ; (2) that it is most probably an offshoot from the same 

 magma as that which supplied the Inchcolm rock. It differs from 

 the Bathgate rock." 2 



Through the bed of picrite run veins of a very different 

 character. The vein-rock is holocrystalline and consists of : — 



(1) Augite, well preserved, white, brown, and pinkish in colour, 



in large crystals. 



(2) Plagioclase-felspar, in large crystals showing twin-striation. 



Originally the plagioclase formed a large proportion of the 

 rock, but it is now to a great extent changed into a turbid 

 white mineral. 



(3) A greenish-yellow, secondary mineral, which has scarcely the 



shape of olivine — it may be bastite, but is not unlike the 

 green mineral found in beds Nos. 4 and 6 (mica-porpbyrite), 

 and possibly it may be a peculiar variety of augite. 



(4) There are several brown and greenish dichroic crystals. Some 



of these are apparently mica, some are hornblende. About 

 others I feel considerable doubt — possibly they may be an 

 actinolite. 



(5) Iron oxide is fairly abundant. 



(6) Zeolites occur, as in the rock of bed No. 6. 



Mr. John Henderson described one of these veins, probably the 

 same as that from which I brought away a specimen, as a vein or 

 thin dyke cutting the ' trap ' and passing obliquely from below to 

 the surface. 3 Mr. Goodchild considers them to be segregation- veins. 

 Perhaps they may be described as dolerite, more especially as they 

 bear a strong resemblance to the igneous rocks of No. 11, which 

 overlies the picrite and is certainly an ophitic dolerite. 



A reference to the Geological Survey Map (Sheet 32, Scotland) 

 will show that the Corstorphine Hill mass of igneous rock projects 

 northward towards the Forth, and the cutting which I am de- 

 scribing is situated nearly at the northernmost end of the patch 

 mapped. Accounts of the igneous rock of Corstorphine Hill are given 

 by Mr. Allport and Mr. Teall, 4 and their descriptions agree well 

 with a micro-section cut from a specimen which I obtained in Sir 

 James Maitland's large quarry, south of the Queensferry Boad, 



1 See A. Geikie, Trans. Boy. Soc. Edinb. vol. xxix. (1879) p. 507 ; also 

 Teall, op. supra cit. 



2 A. Geikie, op. supra cit. 



3 Trans. Geol. Soc. Edinb. vol. vi. part v. (1893) p. 299. 



4 Quart. Journ. Geol. Soc. vol. xxx. (1874) pp. 557-558 : ' Brit. Petrogr ' 

 1888, p. 190. " *.' 



