10 
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 made, (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 fi'equently 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 ivitJwut 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 vdth it so intimately that the one passes in- 
sensibly into the other. For 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 white. 
Is it quite certain that mica pseudomorphoses kyanite (disthene) ? 
I do not think 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 
