16 



THE GEOLOGIST. 



ties ; and in order to solve them we must, in the first place, seek to 

 ascertain how envelopment is produced. 



When a mineral crystallizes, the substance which it envelopes 

 remains sometimes amorphous. This, for example, is what takes 

 place in the sand which is found in the rhombohedrons of calcite from 

 Fontainebleau. It is the same with macle, (andalusite,) which, 

 according to M. Durocher, has retained a part of the schist in the 

 midst of which it formed. But the mineral enveloped in another 

 which is crystallized, has most frequently been crystallised itself. If 

 we consider two minerals in those conditions, we must distinguish the 

 case in which their crystals are independent, and that in which they 

 are symmetrically arranged. 



1st, Envelopment without Symmetrical Arrangement. — The first case 

 is the simplest and also the most frequent. Generally, when two 

 crystallized minerals envelope each other, their crystals have any 

 direction with regard to one another, and are independent. 



Thus magnetite in hornblende, chlorite in calcite, mica in augite, 

 in hornblende, in orthoclase, and in the felspars, are most frequently 

 in crystals completely independent of the minerals in which they have 

 formed themselves. 



As long as the enveloped mineral is found in crystals clearly isolated 

 and not numerous, no confusion is possible between envelopment and 

 pseudomorphism. On the contrary, we find ourselves in the presence 

 of the greatest difficulties as soon as the enveloped mineral becomes 

 sufficiently abundant to disguise, as it were, the enveloping mineral ; 

 or when it is associated with it so intimately that the one passes in- 

 sensibly into the other. Tor example, garnet has been considered 

 pseudomorphic after idocrase because it is observed sometimes in its 

 interior ; and this is, indeed, what I had the opportunity of verifying 

 in the collection of M. Wizer, at Zurich. But it is necessary to remark 

 that the idocrase is, in its turn, enveloped by the garnet. Although 

 it is very easy to conceive the metamorphism of these two minerals, 

 since they have nearly the same chemical composition, I think we 

 should only admit it if it were clearly established that the garnet can 

 substitute itself entirely in the place of the idocrase. 



We should also observe the same reserve with regard to iolite, 

 (dichroit, cordierite,) and mica ; for iolite, whenever it bears no trace of 

 alteration, often covers itself with very numerous scales of mica, under 

 which it so disappears, that it is necessary, in order to recognize it, 

 to examine its fracture in a plane perpendicular to the scales. In the 

 variety of Amity (Maine) which has been designated under the name 

 of chlorophyllite it is easy to establish that the large scales of 

 green mica are very close together, and that they alternate with the 

 bluish while. 



Is it quite certain that mica pseudomorphoses kyanite (disthene) ? 

 I do not t hink so ; it has merely seemed to me that kyanite frequently 

 enveloped a greater or less proportion of mica, which was mixed with 

 it, and into which it might even pass. But there is nothing in this 



