472 PROF. W. S. BOULTON ON A MONCHIQTttTE INTRUSION [Nov. I9I I, 



monchiquite near the contact usually shows the dark-brown, glassy 

 matrix already described as characteristic of the vicinity of 

 xenoliths. 



Allotriomorphic crystals of olivine makeup perhaps the bulk 

 of the rock, showing the characteristic cracks, but with none of the 

 original mineral left. In its place is a colourless or grey substance 

 (carbonate) l with fibres and scales of serpentine and iron-oxide. 

 Beautiful examples of stellate and dendritic growths of magnetite 

 occur. 2 



Associated with the olivine is the mica-like mineral, which 

 is so conspicuous in hand-specimens of the nodules. Micro- 

 scopically, it is dark green or brown, often with a pronounced 

 s chiller or bronzy lustre, and a strong cleavage, so that it can 

 be peeled off by a knife into flakes which are very soft and brittle, 

 and easily crush into a greenish-yellow powder. 



Thin sections are yellow and green or pale reddish-brown by 

 transmitted light, in nearly ail cases distinctly pleochroic, the 

 colour being green or bluish-green for vibrations parallel to the 

 short axis of the polarizer, and yellowish for vibrations at right- 

 angles. The cleavage is very marked, and extinction invariably 

 takes place parallel to it. Cleavage-flakes, examined separately, 

 show a variety of colour by transmitted light, from bright yellow 

 to green and brownish-red. A yellowish-green flake, with fine 

 striations and faint granular patches and slight dichroism, gave in 

 convergent polarized light the acute bisectrix of an interference- 

 figure, with a small optic axial angle, and with the optic axial 

 plane disposed parallel to the length of the flake. Another flake, 

 with clusters of very minute yellow-brown granules, showed fine 

 striations along the plane of main cleavage, and between crossed 

 nicols a marked fibrous or rod-like structure, with the fibres 

 polarizing brilliantly, and in separate bundles. 



The double refraction is strong, the interference-colours belonging 

 to the second order. In one or two places, the habit and optical 

 properties generally show a striking resemblance to biotite. 



The rods, fibres, and lamellae appear to start from one transverse 

 crack, and extend perpendicularly to the next transverse crack. 



There seems to be no doubt that this substance is an alteration- 

 product of the olivine, and it closely resembles the mineral 

 iddingsite, regarded by some mineralogists as a definite variety 

 of serpentine. 



It is just possible that ba s tit e-pseudomorphs after a rhombic 

 pyroxene occur in these nodules, for a few patches of the paler 

 variety of the green substance suggest it ; but, so far, no satisfactory 

 determination has been made. 



Allotriomorphic plates of pale-green augite, often partly or 



1 The finely-powdered rock, when treated with very dilute hydrochloric acid, 

 shows, under the microscope, a vigorous reaction in the fragments of this 

 altered olivine. 



2 See J. W. Judd, ' On the Tertiary & Older Peridotites of Scotland ' 

 Q. J. G. S. vol. xli (1885) p. 382. 



