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The first British rock definitely recognised as belonging to the picrite 

 family is the augite-picrite of the Island of Inchcolm in the Firth of Forth. 

 Specimens of this rock were collected by Mr. ADY, and described by 

 Dr. A. GKIKIK (I) in 1880. The rock has since been described, and figured 

 in COI.KS' " Studies in Microscopical Science." (2) It forms a conspicuous 

 mural cliif of a rich brown colour and singularly barren aspect, on the 

 cast side of the landing place by the abbey. 



In hand specimens the rock is seen to be moderately coarse in texture 

 and dark green in colour 011 a freshly exposed surface. Grains of olivine 

 and glistening cleavage surfaces of a black mica may be easily detected with 

 a lens ; also light coloured greenish patches which vary in abundance in 

 different specimens, and which probably represent pseudomorphs after 

 felspar. The examination of a large series of slides prepared from different 

 portions of the mass shows that the rock is by no means uniform in 

 composition. The constituents are olivine and its serpeiitinous pseudomorphs, 

 augite, brown hornblende, biotite, felspar and pseudomorphs after felspar, 

 iron-ores and apatite. 



Olivine occurs in crystals and as grains. In some slides it is wonderfully 

 fresh, but as a rule it is traversed by irregular anastomosing strings of honey 

 yellow or bluish green (aquamarine) serpentine, the former colour pre- 

 dominating. In some specimens the olivine is wholly replaced by serpentine 

 and, when this is the case, the honey yellow variety usually forms the main 

 mass of the pseudomorph , passing over sometimes at the margin into the 

 green variety which fills up the interstices between the other constituents 

 of the rock. 



The augite varies in tint from nearly colourless to a pale reddish brown. 

 The tint frequently varies in one and the same individual, sometimes 

 gradually and sometimes abruptly. In the latter case sections parallel to 

 the vertical axis occasionally show the " hour-glass " structure (see Figs. 3 

 and 4, Plate V.), and sections more or less at right angles to this axis show 

 a central kernel of nearly colourless augite with an outer zone of the coloured 

 variety. The coloured variety is markedly pleochroic. It is difficult to 

 define the pleochroism with precision from the material at the writer's 

 disposal, but some idea of its character may be given by saying that sections 

 approximately at right angles to the vertical axis of the prism change from 

 a pale yellow when the vibration plane of the polariser lies parallel to the 

 optic axial plane of the crystal, to a brownish violet when the stage is rotated 

 through 90 (see Figs. 1 and 2, Plate VII.) This renders it highly probable 

 that the pleochroism is the same as that of certain augites rich in titanium. 



The augite of the Inchcolm rock sometimes shows an approach to definite 

 crystalline form ; but more commonly it is interfered with by the other 

 constituents, including the felspar. 



(1) Trans. Roy. Soc. Edin.. Vol. XXIX., p. 506. 



(-2) For June 10th and 17th, 1832. See also JUDD. Q.J.G.S., Vol. XLL, p. 400. 



