1032 A TREATISE ON METAMORPHISM. 



Springs of Yellowstone National Park. At this place there issues a 

 vast amount of water, and great, brilliant travertine deposits, consisting of 

 almost pure calcium carbonate, have been built up.° In this vicinity are 

 Tertiary sediments which include limestones. There can be no doubt in 

 this case that the waters are heated hy the igneous rocks, and that the 

 source of the material which is dissolved by the heated waters is the 

 intruded sediments and not the igneous rocks. The case is particularly 

 conclusive, because the geyserite deposits in the adjacent basins where 

 sedimentary rocks are not close at hand consist of silica. If the heat of 

 the igneous rocks can be so effective in promoting the solutions of vast 

 quantities of calcium carbonate in the adjacent sedimentary rocks, may it 

 not be equally effective in dissolving a sufficient amount of gold, silver, 

 and copper, so that in a deposit a fraction of an ounce of gold, a few 

 ounces of silver, or a few per cent of copper may be found ? 



It is also certain that many ore deposits derive their metalliferous 

 content in part from the intrusive and in part from the intruded rocks. 

 Probably this is the most frequent of all cases. To give an estimate of the 

 relative amounts of metalliferous metals derived from the original igneous 

 rocks and from the secondary rocks is quite impossible. 



Recently there has been a strong tendency among many geologists to 

 maintain that the metals in ore deposits are mainly or wholly derived from 

 almost immediately adjacent igneous rocks. As already intimated, this is 

 undoubtedly true in many cases. For instance, where there is a great 

 complex of aqueous and igneous rocks of many different kinds and the 

 ores are always intimately associated with one particular kind of ig'neous 

 rock and no other, it is very probable that the source of the metal is the 

 intimately associated igneous rock. Such cases are rather numerous. An 

 excellent illustration is furnished by the copper-nickel deposits of Sudbury, 

 which are invariably associated with norite. Another case in which it 

 is believed that the ore is mainly derived from the igneous rocks is that of 

 the copper deposits of the Lake Superior region, which occur in amygda- 

 loids and conglomerates, but a portion of the copper was probably leached 

 from the conglomerates. This belief is not based upon the fact that these 

 ores, occur to a considerable extent within openings of the amygdaloids, 



"Hague, Arnold, Weed, W. H., Iddings, J. P., Yellowstone National Park: Geol. Atlas U. S., 

 folio 30, U. S. Geol. Survey, 1896. 



