400 



THE GEOLOGIST. 



mineral enveloped in the following ones. Althongli tlie minerals 

 enveloping or enveloped are generally inorganic, they may also be 

 organic, and these are also given in the table. A particular attention 

 has, besides, been given to the examination of minerals in which any 

 metamorphism has been noticed, and we shall see further on that 

 this supposed metamorphism often finds a perfectly natural 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. 



Envelo]pment 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 Hme 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 bro^vn. 



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 opaque zones succeed each other in considerable 

 numbers. Sometimes it is the same with felspar, and particularly 

 with the orthoclase of the porphyritic gi-anite of the Yosges.* 



* " EecliercliGs sur les Eoclies Globtileuses" (Meinoires de la Socicte Geolo- 

 gique, 2nd serie, tome iv., p. 301). 



