476 MOXCHlQTJIrE 3 NTKTTSION IN OLD RED SANDSTONE. [Nov. 19H, 



I wish to express my indebtedness to Dr. Elett for very kindly 

 advice, for examining some of my rock-sections, and for the loan of 

 specimens from Colonsay ; to Mr. Herbert H. Thomas for calling 

 my attention to the Colonsay rocks ; and to Prof. Sollas for the use 

 of the apparatus in the University Geological Laboratory at Oxford 

 for photographing the rock-sections. 



EXPLANATION OF PLATE XXXVII. 



Fig. 1. Slide M 411. Showing a nearly colourless augite (ehrome-diopside)- 

 phenocryst with corrosion-border and added zone of purple-brown, 

 augite. The rest of the field is the dark-brown inoncbiquite matrix, 

 with small crystals of augite and olivine. X 20 diameters. 



2. Slide M 404. Monchiquite with plates of biotite, the two near the top 



of figure enclosing an olivine-crystal. In the left upper corner and 

 on the right are larger corroded and decomposed olivine-crystals. 

 The ground-mass is dark grey, with octahedra of iron-ore and small 

 olivine-crystals. X 20 diameters. 



3. Slide M 403. Monchiquite, showing in the lower part of the figure a 



reddish-yellow biotite-crystal, and to the right of the centre a dark 

 reddish-brown chromite-granule. The dark-grey ground-mass encloses 

 small prisms of pale-brown augite, with a few small olivine-crystals 

 and granules of iron-ore. X 20 diameters. 



4. Slide M 401. In the lower right-hand corner is part of an Old-Red- 



Sandstone xenolith (measuring half an inch across). Three lapilli of 

 dark-brown and reddish-brown palagonitej enclosing crystals of augite 

 and small round vesicles, make up half the field. They are embedded 

 in a colourless or light-grey matrix, consisting mainly of carbonate, 

 with small lapilli of glass and granules of quartz. X 20 diameters. 



.£).. Slide M 420. Junction of the Old lied Sandstone xenolith, showing 

 the local development of basic glass. On the left is part of the 

 xenolith, with corroded border ; in the centre is dark-brown glass 

 enclosing pale-brown augite-crystals and vesicles ; on the right is the 

 monchiquite with analcite ground-mass. X 20 diameters. 



6. Slide M 415. Cognate xenolith of picrite. To the left and in the 

 centre is a large plate of very pale yellowish-green chrome-diopside, 

 enclosing serpentinized olivine. Near the top is another area of 

 decomposed olivine, partly surrounded by augite ; while the dark 

 patch on the right is deep greenish-yellow translucent chromite. 

 ;X 20 diameters. 



Discussion. 



Dr. J. W. Evans remarked on the similarity of the large augite 

 mentioned in the paper with that described by Heddle from the 

 volcanic neck at John o' Groats. He also referred to the fact 

 that the analcime ground-mass of monchiquites represented the 

 only case known in the consolidation of rock-mag tn as, in which a 

 eutectic residue was a definite chemical compound instead of a 

 mixture. The low melting-point of analcime, which was less than 

 that of any other original mineral constituent of igneous rocks, was 

 due to the fact that it contained water ; and it was the presence 

 of water in the magma that determined the occurrence of 

 monchiquite, rather than that of a basic rock containing plagioclase 

 and nepheline. 



