SALMON — RESEARCHES ON PSEUDOMORPHS. 



401 



The idocrase of Arendal, according to Mr. Gr. Leonhard, presents 

 a series of crystals whicli fit well with, each other; their lustre 

 diminishes as they recede from the centre, and it may even occiir 

 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 very well indi- 

 cated by the parallel zones of ripidolite (hgn 2). 



Fig. 2.— Parallel Zones of Ripidolite. 



A very small quantity of foreign matter, or a slight alteration in 

 the structure 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 tourmaline, 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, leucite, 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 crystal- 

 lization effected in a liquid, or in a medium of variable composition, 

 or to a severance occuiing between the parts at the moment of crys- 

 tallization, 



Recvprocol envelojwient. — 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 with kyanite and 

 staurolite. 



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 Mourne, in Ireland, the orthoclase 

 impresses itself on the quartz, while in the cavities of this same peg- 



VOL. III. 3 E 



