116 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



6. Although those modifications of force known as crystal- 

 ization and chemical affinity act most readily in fluid masses, yet 

 they are able to act also in solids. 



7. All rocks, however solid they may appear, possess more 

 or less plasticity when subjected to long-continued pressures, 

 as may be very clearly seen in contorted sections of strata, and 

 also in the distortion of fossils found in shale and slate. 



8. From the surface of the earth (or from points very near 

 the surface) downwards, there is everywhere a considerable and 

 progressive increase of temperature. 



9. So far as our knowledge of the interior of the earth 

 goes, there is always present a large proportion of water in 

 various forms. 



Genesis of Ore-deposits. In studying this part of our subject, 

 after defining and fixing the sense in which the term ore is used, 

 we have to consider the nature of the containing or "country" 

 rock, the manner in which a locus for the deposit has been 

 prepared, and finally, the way in which that locus has become 

 filled with ore-matter. 



By the term ores we understand either native metals while 

 still associated with their matrix or containing rock, or else such 

 chemical compounds of metals proper as are usually worked for 

 commercial ends. 



Some ore-deposits are, quite obviously, of the same age, or 

 approximately so at least, as their containing rocks ; thus, a bed 

 of iron ore of aqueous origin lying between two beds of sand- 

 stone would be somewhat newer than the lower bed, and some- 

 what older than the upper. But the great majority of ore-deposits 

 are of much later origin than their containing rocks. We might 

 therefore consider ore-deposits under the following heads, viz : 

 (a) Contemporaneous deposits, comprising chiefly true ore-beds ; 

 (h) Secondary deposits, impregnations, segregations, &c, in rock- 

 masses, veins, lodes and irregular deposits ; (c) Detrital deposits, 

 or natural mechanical concentrations. 



Of course these classes overlap to some extent ; thus a true 

 bed may be altered by later segregations or withdrawal of 

 material, while even detrital deposits are often modified by 



