PRINCIPLES CONTROLLING ORE DEPOSITS 753 



in the precipitation of ores. If the contents of half a dozen test 

 tubes filled with solutions chosen at random be dumped together, 

 a precipitate is almost sure to form. And just so sure as under- 

 ground waters come from this source and that source and mingle 

 in the trunk channels of underground circulation, just so surely 

 are precipitates formed. Fourth, in the formation of an ore 

 deposit the wall rock may contribute a solution which precipitates 

 a metal, or it may contribute a metal which is precipitated by a 

 solution. Consequently an ore deposit may be confined to a 

 particular horizon where there is a certain rock. For instance, 

 lead and zinc are very generally associated with limestone, 

 and the sandstones or other rocks above or below are very 

 likely to be deficient or nearly devoid of these metals. To 

 a less extent other ores show a decided preference for lime- 

 stone as compared with other rocks. The explanation may be 

 that the limestone itself furnishes the material ; and this is 

 believed to be the fact in various cases. The explanation may 

 be that the limestone furnishes a precipitating agent to solutions 

 derived from other rocks. It may be that the limestone because 

 of its ready solubility furnishes large openings in which big 

 deposits may be formed. Finally the explanation may lie in 

 the combination of two or more of these factors. I have no 

 doubt if we consider the whole world each of these factors is 

 important, and that in some cases all of them cooperate. As a 

 result of the combination of the various factors above considered 

 a porous rock or an opening once in a million or ten million 

 times receives enough of the metallic materials in solution so 

 that a fraction of an ounce of gold per ton, or a few ounces of 

 silver per ton, or a few per cent, of copper or some other metal, 

 or a large per cent, of iron, will be precipitated ; and we call 

 the material an ore deposit. An ore deposit it is from an eco- 

 nomic point of view. From a geological point of view it is usually 

 to a far greater extent quartz and calcite and other gangue 

 minerals. 



I wish now to go a little further and consider the fissure on 

 the slope shown in this chart (Fig. 6), both in the past and the 



