150 CLASSinCATION OF THE PUBLIC LANDS. 



its age, for valuable deposits are found in rooks of all ages, from the 

 Tertiary to the most remote. 



In a particular district, however, the ores may show a preference 

 for certain rocks. The favorable rocks may be those that contain 

 minerals in a finely disseminated condition which may be valuable if 

 they are anywhere sufficiently concentrated. Copper, for example, 

 seems to be a constituent of some basic igneous rocks. Heady replace- 

 ability, or the power of reacting vigorously with mineralizing solti- 

 tions, may be the determining favorable factor ; and where the typical 

 deposits of a district are replacements or contact-metamorphic de- 

 posits calcareous sediments are usually more favorable than others. 

 Again, the presence of some constituent which acts as a precipitant 

 may determine the concentration of a valuable mineral ; organic mat- 

 ter probably precipitates gold under some conditions, and carbona- 

 ceous slates appear to be a common country rock of gold deposits. 

 Finally, some purely physical feature may be determinative. The 

 hard rocks of a district may be fissured to form breccias in which the 

 ore-bearing solutions can circulate and deposit, while fissures in soft 

 rocks would be clogged with impermeable gouge; and fissures which 

 are large and persistent in massive rocks might ramify, on entering 

 fissile rocks, into a multitude of small slips parallel to the bedding or 

 cleavage. 



Intrusions and metamorpMsm. — Although the nature and degree 

 of the relation between igneous intrusion and ore deposition are still 

 moot points, it is an established fact that ore deposits are especially 

 abundant in the vicinity of intrusive contacts. Apart from strati- 

 form deposits-r-such, for example, as those of iron oxide — ore bodies 

 so remote from intrusive rocks as the lead and zinc deposits of the 

 Mississippi Basin are rather exceptional, though not so exceptional 

 as to make absence of intrusives a sufficient ground for classification 

 of land as nonmineral. The general similarity in the distribution 

 of ore deposits and of igneous rocks is particularly notable in the 

 Western States. The presence of igneous intrusions must therefore 

 be considered favorable, in general, to the deposition of ores. 



Ore bodies related to intrusion are found both in the intrusive 

 rock itself and in the surrounding sedimentary rocks. Those which 

 are not contact-metamorphic deposits in the strict sense are not more 

 likely to be at the immediate contact than at a considerable distance 

 from it. The central portion of a very large batholith is likely to 

 be barren, but the peripheral portion is commonly ore bearing, and 

 all of the denuded portion of a small batholith or stock is likely to 

 contain ore deposits, for none of it is far from the contact. Large 

 dikes also form the country rock of many ore deposits. The ores are 

 formed in the rocks cut by the intrusive, not only within but beyond 

 the zone of contact metamorphism. 



