40 MR. H. W. M0NCKT0N ON A PICRITE [Feb. 1 894, 



They are very similar one to another. There are a few porphyritic 

 crystals of a green mineral which may to some extent replace olivine ; 

 hut I think that in most cases it more probably replaces augite, 

 The shape of the crystals is not quite that of olivine, and in the 

 slide from bed No. 4 there are one or two crystals not so much 

 altered as the rest, and one of them seems to extinguish at an 

 angle of about 45°, and is, I think, certainly augite. Perhaps some 

 of these green crystals, especially some of those in the slide from bed 

 No. 6, may be alteration-products after bastite. Some of them 

 are bordered with small flakes of brown mica (no doubt secondary), 

 and there is a great deal of brown mica in the groundmass, the 

 flakes of which are minute in bed No. 6, while in bed No. 4 they 

 are often 0*006 inch long. There are some small crystals of augite, 

 and a great deal of very much altered plagioclase-felspar in small 

 lath-shaped crystals. Many zeolites occur all over the section, 

 and there are a few crystals which I think may be epidote. 



At one spot in the micro-section from bed No. 6 there is a very 

 pretty group of zeolites at the base of the igneous rock, where it 

 rested on the shale of bed No. 5. It is not very easy to name this 

 rock. Perhaps ' mica-porphyrite ' is the most appropriate appellation, 

 although the rock can scarcely be said to contain porphyritic felspar : 

 it is somewhat nearly allied to kersantite. The mica-porphyrite 

 lies very evenly hctween the beds of stratified rocks, but the great 

 induration of the shales above and below it shows that it is probably 

 intrusive. 



[When I sent this paper to the Society I was not aware that the 

 cutting had already been described by Mr. John Henderson and 

 Mr. J. G. Goodchild. 1 The former of these authors mentions the 

 igneous rock which I have called mica-porphyrite as ' intrusive 

 greenstone.' The picrite and overlying basalt he describes as a mass 

 of greenstone dipping conformably with the shales, and having an 

 apparent bedding and dip to the west. This apparent bedding, due 

 to planes of jointing, is very marked. Mr. Goodchild points out 

 that a portion of this mass of greenstone is picrite : he does not 

 deal with the mica-porphyrite.] 



The picrite is No. 10 of the section (see p. 39), and it forms the 

 sides of the cutting for some distance. The greater portion is soft 

 and granular, but hard parts may be found here and there. The 

 planes of jointing are fairly parallel to the stratification of the shales, 

 etc., which underlie it. 



Under the microscope it is seen to be composed of: — 

 (1) Serpentinized olivine, in more or less rounded grains of very 

 different diameters — one of medium size measures 0'08 X 0-04 inch. 

 The olivine has been altered into a mineral of a brilliant j T ellow 

 colour, with a somewhat parallel fibrous structure, and round the 

 edges of the grains a further alteration into a bluish-green and more 

 markedly fibrous mineral has taken place. This mineral is dichroic, 

 changing from bluish-green to yellow. Sometimes, especially 

 1 Trans. Geol. Soc. Edinb. vol. vi. part v. (1893) pp. 297, 301. 



