Park. — The Deposition of Mineral Matter. 37 



(3.) Electro-chemical action. 

 (4.) Chemical precipitation — 



(a.) By contact with other mineralised solutions. 



(b.) By gaseous emanations. 

 (5.) Absorption of metals from dilute solutions by silica, 



clays, and porous substances. 



The dissolution and deposition of mineral matter from 

 aqueous solutions must necessarily be governed by physico- 

 chemical laws. It is therefore reasonable to assume that the 

 prevailing geological conditions in each case will determine 

 the forces or processes that will be brought into operation. 



The dissolving-power of water is enormously increased by 

 heat and pressure, and it has been proved experimentally 

 that water and water-vapour at high temperatures and pres- 

 sures are capable of dissolving almost all known rocks and 

 metals. Hence water will possess its greatest solvent power 

 at the greatest depth reached by it, whether it is disengaged 

 from a cooling igneous magma, or exists as a deep-seated cir- 

 culating current. In the first case the water and vapour will 

 gather their mineral contents from the parent magma, either 

 in whole or in part, and in the second case from the rocks 

 through which the channels chance to pass. 



The hot mineral-laden solutions will naturally tend to 

 ascend, and in ascending will gradually part with heat and 

 become subject to less pressure. The substances which were 

 dissolved only at the greatest temperature and pressure will be 

 the first to pass out of solution ; and thereafter, as the solu- 

 tions ascend, with decreasing temperature and pressure, the 

 dissolved substances will be deposited in the inverse order of 

 their solubility. The most difficultly soluble substances will 

 be the first to go out of solution, and the most easily soluble 

 the last. Thus when the ascending waters reach the surface 

 we should only expect to find in solution the easily dissolved 

 alkaline silicates, carbonates, and sulphates. 



It is notorious that hot mineral springs do not deposit sul- 

 phides at the surface. The cinnabar which has been and is 

 still being deposited in the sinters at Ohaeawai hot springs in 

 New Zealand is being formed from gaseous emanations, and 

 not from the mineral waters. This is also probably true of 

 the cinnabar-deposits at Steamboat Springs and Sulphur Bank 

 in America. At Ohaeawai, Rotorua, and everywhere through- 

 out the volcanic regions of New Zealand solfataric and fuma- 

 rolic action are intermittent phases of the same pipe or vent. 

 In many cases, however, hot springs and fumaroles exist side 

 by side. 



The weathering and oxidation of the outcrops of metalli- 

 ferous lodes bv meteoric waters, followed bv the transference 



