400 THK GEOLOGIST. 
mineral enveloped in the following ones. Altliougli the minerals 
enveloping or enveloped arc generally inoi'ganic, they may also be 
organic, and these are also given in the table. A particular attention 
lias, besides, been given to the examination of minerals in which any 
mctamorphism has been noticed, and we shall see further on that 
this sujjposed mctamorphism often finds a perfectly natm-al explana- 
tion in envelopment. 
The table shows at once that the enveloping minerals, as well as 
the enveloped minerals, may appertain to every family of the mineral 
kingdom. It informs us in addition, as to the most habitual associa- 
tion of the divers minerals, and it enables us to embrace it at a glance. 
Some interesting peculiarities merit pointing out in the first place. 
Envelopment of varieties of one same mineral. — Envelopment may 
easily be established, not only between different minerals, but also 
among varieties of the same mineral. Then the name of this mineral 
has been inscribed both in the columns of enveloping and enveloped 
minerals. The following are some examples. 
Among the best crystallized bodies, as the diamond, in the midst 
of the most limpid sorts there are dull or even completely black 
parts, which form in certain cases species of fixed asteries. 
Hyalin quartz often encloses independent crystals of quartz equally 
hyalin. In Iceland-spar M. Des Cloizeaux has observed crystals of 
carbonate of lime which are perfectly distinct from it. The horn- 
blende of crystalline schists is often formed of common (aluminous) 
hornblende enveloping actinote. 
The silver- white mica of granite frequently encloses another mica 
which is blackish or pinchbeck brown. 
Tourmaline presents, particularly, very distinct varieties in one 
same crystal ; thus, that at Chesterfield is green at the exterior, and 
a fine rose colour in the interior. On the other hand, the opposite 
may also happen, as is shown by a tourmaline of Mursinsk, in Siberia, 
belonging to the collecion of Mr. Damour. In certain tourmaline 
crystals we even observe several alternations. 
The leucite of Vesuvius appears in small globules with concentric 
zones, in which a transparent zone is comprised between two opaque 
Fig. 1. — Leucite. 
zones (Fig. 1). In the large leucite crystals of Roccamonfina, the 
transparent and opaqiie zones succeed each other in considerable 
nvimbers. Sometimes it is the same with felspar, and particularly 
with the orthoclase of the porphyritic granite of the Vosges.* 
* " Rechcrclies sur les Roches Globuleuses" (Memoii-es de la Societe Geolo- 
gique, 2nd serio, tome iv., p. 301) . 
