Modes of Occurrence of Ores. 27 



can be distinguished by no decisive character from those which 

 are assumed to have been produced subsequently to the rocks 

 they traverse, but, on the contrary, present an uninterrupted 

 series with them,* it is better to leave this classification aside, 

 and to distinguish the repositories of useful minerals, when it 

 is necessary to do so, entirely by their form, and by their re- 

 lations to the surrounding rocks, and to their structure, that 

 is, without reference to their period of formation, or to their 

 relative age. 



Disseminated Ores and Stochcerh, fyc. — It is the case with 

 some useful minerals, that they are not confined to a space 

 of determinate form, but that they occur disseminated or 

 distributed in smaller or larger imbedded portions, in certain 

 portions of mountain-masses. Tinstone and native gold are 

 remarkable examples of this mode of occurrence. The rocks 

 in which these metals are met with, are, in respect to the 

 former, granite and porphyry, and more rarely gneiss ; for 

 the latter, granite and some sandstones, and probably also 





* Some geognosts, probably from noticing this contradiction, and from 

 having perceived the undeniable simultaneous formation of these veins with 

 the rocks themselves, endeavour to separate the contemporaneous veins from 

 the true veins, that is, from tabular repositories, which, as is assumed, have 

 been formed at a later period than the surrounding rocks, therefore in actual 

 previously existing open fissures ; and, in order to establish this distinction, 

 they avail themselves of various characters, which are partly derived from 

 the form of these repositories, partly from their contents, and partly from 

 their relations to the rocky masses. 



It is true that these characters belong to several of these so-called con- 

 temporaneous veins, but they are also met with in many of those regarding 

 which it is asserted that they have been produced at a later period than the 

 traversed rocks : while, vtee versa, those properties by which the latter are 

 distinguished, are to be found in the former in greater or less perfection. 

 When, in a scries of veins of this description, we compare the neighbouring 

 members, we find but few differences, but, when we compare more distant 

 ones, the difference is so great that we can hardly discover one character in 

 which they agree. This is the nature of these repositories, and in this con- 

 sists the character of the uninterrupted series which they exhibit. The at- 

 tempt to discover decided marks of distinction between beds, contempora- 

 neous veins, and real veins, that is, those presumed to be of subsequent for- 

 mation to the rocks in which they occur, will probably, therefore, never suc- 

 ceed, if we except from the latter those which have been formed in more or 

 less considerable tabular hollows, of actual fragments and rolled masses, of 

 bone fragments, \c. or which have been produced on active volcanos in ac- 

 tual fissures, all of which arc not to be included among the original reposi- 

 tories of minerals now under consideration. 



