British Mineralogy. 333 



globe, present themselves under similar conditions, have the same 

 associated minerals along with them, and that the eruptive rocks 

 in which they occur, whenever the age of their intrusion could 

 be satisfactorily ascertained, frequently, if not always, corre- 

 sponded in geological chronology. This is more particularly the 

 case with such minerals as have not been produced by a subse- 

 quent metamorphic action* (or alteration of previously existing 

 species), and which maybe assumed to be present in the rock in 

 the unaltered condition in which they originally were developed 

 in situ f. 



Some, but comparatively few, minerals must be regarded as 

 common to a variety of situations and circumstances, frequently 

 occurring as normal constituents of very different rocks, although 

 not really characteristic of any one precise rock or formation. 

 Such minerals at times owe their appearance to chemical reac- 

 tions and processes quite dissimilar to those which at other times 

 have caused their formation J. 



Other minerals, the chemical elements of which, being com- 

 mon to the mass "of the rock-matrix in which they occur, may 

 merely indicate by their presence that they have been formed by 

 the combination and rearrangement of these elements in situ, 

 effected by the action of heat, water, or other agency. Epidote 

 is generally an excellent example of this class, being most fre- 

 quently found developed in rocks at their points of contact with 

 eruptive intrusions, which seem to have affected the parts of the 

 rock in juxtaposition sufficiently to cause its constituents to 

 unite and form this mineral. In other cases such effects appear 

 to be frequently brought about by the agency of water, — thus, 

 for example, Vivianite, by the union of its constituents, phos- 

 phoric acid with oxide of iron &c. 



The eruptive rocks, either directly, by enclosing them in their 

 substance, or indirectly, by metamorphic action and the formation 

 of metallic lodes, have supplied the mineralogist with by far the 



* Melanterite (sulphate of iron), for example, is, when met with in 

 nature, invariably the product of metamorphic action, and may be formed 

 by the alteration of a variety of substances, even organic as well as inor- 

 ganic ; although usually the product of the oxidation of pyrites, its origin 

 is not always referable to any distinct original mineral compound. Many 

 hydrated silicates, some of which in nature are produced directly by igneous 

 agency, may also be found as the result of the action of water on rocks 

 which contain their chemical components, silica, alumina, lime, &c, as in 

 the case of stilbite, scolezite, Heulandite, &c. 



t Most sulphides, arsenides, antimonides and their compounds, some 

 native metals, &c. 



X Iron pyrites, quartz, &c, which may at times be either of igneous or 

 aqueous origin. It seems very probable that even in such cases the more 

 minute study of the characters, especially of physical structure, may lead 

 to means of discrimination. 



