64 
Rev. T. G. Bonn eg — The Lherzolite of the Ariege. 
rough-looking, something like timt of augite. Colour a translucent 
rather deep olive green, occasionally slightly inclining to brown, 
in No. IV. a rieh umber brown. Bosenbusch (Mikroscop. Physiog. 
p. 160 ) gives the colours of picotite as yellow to brown, transparent 
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 irregulär 
cracks, which occasionally indicate a rüde cleavage. IV. is less 
rieh 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. II. 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 in I., and the diopside shows a rather smoother texture. 
The cracks in the olivine are often bordered on both sides by 
a finely iibrous Serpentine, the result of decomposition. It remains 
briglit. 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 to 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 across 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 commencemeut 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 from specimens 
from which all the olivine has disappeared, and the alteration into 
serp>entine 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. 
