The quartz sometimes shows bubbles containing liquid, gas, and small heavy crystals, probably of ferric oxide, as in the Clifton-Morenci district of Arizona. It is clear that the ore-bearing solutions in these cavities, before the crystallization of the heavy mineral inclusions, held dissolved not only much larger quantities of mineral substances than can be taken up by water at ordinary temperatures, but also a substance like ferric oxide which is entirely insoluble under ordinary cool conditions.
(5) The association of the ores with minerals carrying fluorine and boron, with many silicate minerals, such as garnet, amphiboles, pyroxenes, mica (sericite) and others, and with other minerals which are known to be characteristic developments within or near igneous masses and which are not known to form under weathering agencies at the surface. Various characteristic groupings of these associated minerals are noted. In limestone much of the mass may be replaced by garnet and other silicates in a matrix of quartz. In igneous rock the ore-bearing solutions may have altered the wall rock to a dense mixture of quartz, sericite, and chlorite. Where sericite is dominant, the alteration is called sericitic alteration. Where chlorite is important, it is sometimes called chloritic or "propylitic" alteration. The chloritic phases are usually farther from the ore deposit than the sericitic phases, indicating less intense and probably cooler conditions of deposition. Locally other minerals are associated with the ores, as, for instance, in the Goldfield district of Nevada (p. 230), where alunite replaces the igneous rock. Alunite is a potassium-aluminum sulphate, which differs from sericite in that sulphur takes the place of silicon. In the quartzites of the lead-silver mines of the Coeur d'Alene district of Idaho (p. 212), siderite or iron carbonate is a characteristic gangue material replacing the wall rock.
Quartz in some cases, as noted above, gives evidence of high temperature origin and therefore of igneous association. Jasperoid quartz, as well illustrated in the Tintic district of Utah (p. 235), may show texture and crystallization suggestive of deposition from colloidal solution,—a process which can occur under both cold and hot conditions, but which is believed to be accelerated by heat.
Certain minerals, such as magnetite, ilmenite, spinel, corundum, etc., are often found as primary segregations within the mass of igneous rock. These and other minerals, including minerals of tin and tungsten, monazite, tourmaline, rutile, and various precious stones, are characteristically developed in pegmatites, which are known to be igneous rocks, crystallized in the later stages of igneous intrusion. When, therefore, such minerals are found in other ore deposits an igneous source is a plausible inference. For instance, in the copper veins of Butte, Montana (p. 201), are found cassiterite (tin oxide) and tungsten minerals. Their presence, therefore, adds another item to the evidence of a hot-water source from below.
(6) The occasional existence of hot springs in the vicinity of these ore deposits. Where hot springs are of recent age they may suggest by their heat, steady flow, and mineral content, that they are originating from emanations from the still cooling magmas. In the Tonopah camp (p. 236), cold and hot springs exist side by side, exhibiting such contrasts as to suggest that some are due to ordinary circulation from the surface and that others may have a deep source below in the cooling igneous rocks. This evidence is not conclusive. Hot springs in general fail to show evidence of ore deposition on any scale approximating that which must have been involved in the formation of this class of ore bodies. Much has been made of the slight amounts of metallic minerals found in a few hot springs, but the mineral content is small and the conclusion by no means certain that the waters are primary waters from the cooling of igneous rocks below.
In this connection the mercury deposits of California (p. 259), contribute a unique line of evidence. In areas of recent lavas, mercuric sulphide (cinnabar) is actually being deposited from hot springs of supposed