404 



THE GEOLOGIST. 



figure alternately among tlie enveloping and enveloped minerals. 

 There are even organic substances which envelope certain minerals. 

 These, on the other hand, are found in certain 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, whether they are enveloping or enveloped, the mine- 

 rals belong to all the families of the mineral kingdom. However, 

 silica and the 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 

 very 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 be mitinued.) 



THE EVIDENCES OF THE GEOLOaiCAL AGE AND 

 HUMAN MANUFACTURE OF THE FOSSIL FLINT 

 IMPLEMENTS. 



By the Editor.* 



In tliis notice \Ye commence tlie first of a series of articles descriptive of the 

 geological diagrams, of \Yliich last month we commenced the issue. The 

 notoriety which the discovery of flint implements of human manufacture of 

 Amiens and Abbeville by M. Boucher de Perthes has attained, and the amount 

 of research and investigation noAv going on renders it necessary for us io lay 

 before our readers the ctiief points of tlie proofs of the Immaii workmanship of 



* Being aii illusti'ated explanatory article of i£i-. Mackie's Geological Diagivom, No. VI. 



