﻿64 Ecv. T. G. Bonnpij — The Lherzolite of the Ariege. 



rongh-looking, something like that of augite. Colour a translucent 

 rather deep olive green, occasionally slightly inclining to brown, 

 iu No. IV. a rich umber brown. Eosenbusch (Mikroscop. Physiog. 

 p. 160) gives the colours of picotite as yellow to brown, transpai'ent 

 to opaque ; stating that Pleonaste differs from it in having green 

 tints. If this distinction be correct, the mineral in slides I., II., III. 

 must be Pleonaste. The grains are traversed by rather irregular 

 cracks, which occasionally indicate a rude cleavage. IV. is less 

 rich in picotite than the rest. As the mineral is isometric, it is of 

 course dark between crossed prisms. 



Of the various slides. No. I. is the best for study of the rock, as 

 it is more coarsely crystalline, and shows little or no indication 

 of decomposition. No. 11. shows the grains of the minerals a little 

 more rounded than No. I., and all are much cracked. The olivine 

 appears to bear a rather smaller proportion to the other minerals 

 than iu I., and the diopside shows a rather smoother texture. 

 The cracks in the olivine are often bordered on both sides by 

 a finely fibrous serpentine, the result of decomposition. It remains 

 bright, generally of a pale golden hue, between crossed prisms. 

 No. III. is in structure similar to II., but with more olivine ; here 

 decomposition has advanced further, giving parts of the slide a 

 muddy look, probably due \o faint stains of peroxide of iron ; the 

 serpentine strings are often abundant enough to form a kind of net- 

 work in the olivine, and one considerable crack aci'oss the slide 

 is filled by a feebly double refracting serpentinous mineral. There is 

 a sort of parallel structure perceptible in the direction of the 

 principal cracks, marking a parallelism in the axes of the crystals, 

 and the same is to a slight extent perceptible in the arrangement 

 of the minerals. 



No. IV. gives indications of a structure similar to III., but the 

 change here is much more considerable. A. network of serpen- 

 tinous strings covers almost the whole slide, in many cases invading 

 the other minerals ; the cracks of which are usually free from 

 serpentine in II. and III. In parts the strings seem to coalesce, so 

 as to convert appreciable portions of the slide into serpentine. 

 Here it is interesting to note that clots of opaque dust, doubtless 

 oxides of iron, resulting from the separation of the constituents of 

 the olivine, appear among the strings just as we see them, for 

 example, in the Lizard serpentines. 



These slides therefore exhibit to us, and this is the most interest- 

 ing aspect of the rock, the commencement of the formation of 

 serpentine. In certain serpentines — as, for example, those of Elba, 

 and, as I have recently discovered, of the Lizard — and in some of 

 the olivine bearing gabbros, we can trace the process fi'om specimens 

 from which all the olivine has disappeared, and the alteration into 

 serpentine is complete, to those in which a considerable amount of 

 unchanged olivine is still to be detected. We have thus a further 

 confirmation of the idea, now becoming not unfamiliar to geologists, 

 that much serpentine is an altered olivine rock. 



