ASSOCIATED ROCKS OP THE LIZARD DISTRICT. 917 



polarized light, the clear mineral filling the interspaces of the ser- 

 pentine network proves, as we should anticipate, to be olivine, often 

 very well preserved, and showing brilliant colours, while the ser- 

 pentine (with crossed prisms) is dull milky white, with an indis- 

 tinct fibrous structure in the strings, and often almost or quite dark 

 in the larger patches. The other mineral, which is colourless with 

 ordinary light, is now seen to be of more than one species. One 

 part exhibits the characteristic cleavage of augite ; this occurs in 

 somewhat rounded grains ; very thin veins of serpentine frequently 

 traverse the crystal, following the lines of its cleavage-planes. This 

 mineral does not seem the result of change of the augite, but to have 

 formed in the cracks ; and the generally open condition of the cleav- 

 age-planes rather bears out the idea. The colours of the augite 

 are generally dull; but parts of a crystal occasionally show the usual 

 rich colours, as if the present low tints were due to some subsequent 

 chemical change. The other mineral, also colourless, and showing 

 much the same tints with polarized light, has a peculiar silky aspect, 

 and one well-developed set of close and slightly wavy cleavage- 

 planes. I was at once struck with the resemblance of this mineral 

 to the enstatite in my specimens of lherzolite, and on testing it find 

 it to be orthorhombic, and so true enstatite. I found, however, a 

 crystal or two of ordinary diallage. Besides the above ferruginous 

 microliths, doubtless secondary products, there are a few larger 

 opaque grains of iron peroxide, probably original constituents. I 

 searched the slide for picotite, but could not be certain of any speci- 

 men, though one or two grains resembled an opaque variety of this 

 mineral. One or two cracks, filled with fibrous serpentine, traverse 

 the slide. 



Mullion Cove (no. 8). — "With transmitted light the slide appears 

 to be composed of a great number of small subangular grains of a 

 clear mineral, often associated with aggregated black or brownish 

 dust, and generally clear, rather irregularly oblong crystals, about 

 •02 inch in greatest length, showing a prismatic cleavage, one set of 

 planes being rather strongly developed, all lying in a base of yellow- 

 ish green serpentine. On applying polarized light we find that, as 

 before, the grains are olivine, only that the process of conversion 

 has here advanced a stage further than in the last slide. The other 

 crystals show moderately bright colours : many of them are rather 

 dusky in parts, as if somewhat decomposed ; and the patchy change 

 of the colours with polarized light confirms this. As in the last 

 slide, they seem to have been cracked after crystallization, as 

 though they had been subjected to a strain ; and serpentine has 

 been deposited in the cracks. Some specimens resemble normal 

 augite ; others are nearer to diallage ; other small crystals are ensta- 

 tite. This quite bears out the macroscopic appearance of the rock, 

 which is full of a mineral with a rather silvery lustre, but not ex- 

 actly like ordinary diallage. Here, too, a few small grains very 

 much resemble picotite. In the augitic mineral the planes of prin- 

 cipal cleavage are approximately parallel in many of the crystals, 

 and there are other indications of a flow or pressure structure. This 



Q. J.G.S. No. 132. 3o 



