SALMON — IJHSEARCHES ON TSBUDOMORI'IIS. 
401 
Tho idocrasc of Arcndal, according to Mr. G. Lconliaixl, presents 
a series of crystals Avhiclx fit well with each other ; their lustre 
diminishes as they recede from the centre, and it may oven occur 
that their circumference may be formed of a returning zone of lustre. 
The quartz of the Alps offers, in certain cases, a series of crystals 
which fit each other; and each successive increase is veiy well indi- 
cated by the parallel zones of ripidolite (ligu 2). 
Fig. 2.— Pai'allel Zones of Ripidolite. 
A veiy small quantity of foreign matter, or a slight alteration in 
the structm'B suffice besides to change the aspect of a mineral ; but 
in several of the examples which have just been cited, the density, 
the chemical composition, and all the properties have been com- 
pletely modified. In reality, the envelopment of the tourmahne, of 
the hornblende, of the mica, has taken place between very distinct 
minerals, which yet belong to the same mineral species. 
The envelopment of varieties of one same mineral is easily seen in 
the diamond, fluor, rock-salt, corundum, quartz, augite, hornblende, 
garnet, idocrase, epidote, iolite, felspar, lencite, mica, andalusite, 
kyanite, sphene, tourmaline, topaz, serpentine, wolfram, baryte, gyp- 
sum, calcite, chalybite,. It is rendered perfectly sensible by the 
changes in lustre, colour, transparency, by a mixture of organic 
matters, of metallic oxides or sulphides, of argile, of chlorite, or of 
any other foreign substance ; in a word, it is shown by the very 
smallest differences, whether in the physical properties or in the 
chemical properties. It may be attributed either to a slow crj'stal- 
lization effected in a liquid, or in a medium of variable composition, 
or to a severance occiu'ing between the parts at the moment of crys- 
tallization. 
Reciproccd envelopment. — The envelopment of two minerals is some- 
times reciprocal. Thus quartz envelopes baryte ; and on the other 
hand, the latter envelopes quartz. It is the same vrith kyanite and 
stam'olite. 
More frequently, when two minerals present a reciprocal envelop- 
ment, it is found in different localities ; yet, in certain cases, they are 
not merely in the same locality, but united in the same rock. For 
example, in the crystalline schists of St. Gothard, at times the stau- 
rolite envelopes the kyanite, and at times, on the contrary, is en- 
veloped by it. In the pegmatite of Mom-ne, in Ireland, the orthoclase 
impresses itself on the quartz, while in the cavities of this same peg- 
VOL. III. 3 E 
