404 
THE GH01-.0GIST, 
figiTve alternately among the enveloping and enveloped minerals. 
There are even organic substances which envelope certain minerals. 
These, on the other hand, are found in cei'tain varieties of quartz, 
topaz, and chrysoberyl ; I have shown, moreover, that they exist in 
small quantities in most minerals, sometimes even in those which are 
volcanic. Finally, organized bodies, vegetables or animals, are also 
observed in rock-salt and in amber. 
To sum up, wliether they are enveloping or enveloped, the mine- 
rals belong to all the families of the mineral kingdom. However, 
silica and ihe silicates, carbonates and sulphates are much more fre- 
quently enveloping and enveloped than the sulphides, arsenides, and 
metallic oxides. It is easy, indeed, to understand this ; for while 
the latter minerals are exceptional, the former are on the contrary 
■^ ery frequent, and constitute the greater portion of the terrestrial 
crust. 
As to the origin of enveloping and enveloped minerals, it is very 
variable. The more frequently it is aqueous, but it may also be 
igneous. It is even possible that it may be different for the two 
associated minerals. 
The decomposition of a mineral generally gives rise to an envelop- 
ment. This decomposition is produced by oxygen, water, carbonic 
acid, or indeed by any other chemical agent. It is particularly fre- 
quent in the minerals susceptible of passing to a higher degree of 
oxidation. 
It is the minerals of the abnormal and metalliferous rocks that 
visibly offer the greatest number of envelopments. This is to be 
attributed to their mode of formation, which is usually by successive 
deposits, so one mineral must cover the one that has preceded it. 
This preamble on the envelopment of minerals was necessary for 
the understanding of pseudomorphism, which will now occupy our 
attention- 
(To he continwed.) 
THE EVIDENCES OF THE GEOLOGICAL AGE AND 
HUMAN MANUFACTURE OF THE FOSSIL FLINT 
IMPLEMENTS. 
By the Editor.* 
In this notice we commence tlic first of a scries of articles descriptive of the 
geological diagrams, of which last month we commenced the issue. The 
notoriety which the discovery of flint implements of Inamau manufacture of 
Amiens and Abl)evillc by M. Bouclicr de Perthes lias atlained, and tlie amount 
of research and investigation now going on renders it necessary for us to lay 
before our readers the cliief points of the proofs of the human workmanship of 
* Being an illustrated explanatory article of Mr. Mackic's Geological Diagram, No. VI. 
