THE FORMATION OF DEPOSITS 27 
xlvi, 1890, p. 366), from study of the rocks in Mull, as due to 
solfataric action, and it is known as propylitization. The 
Superheated water may not be pure; it may contain iron 
sulphate ang gold which is deposited in the altered portion 
of the igneous rock, as for example, in the propylitized 
Porphyrites around Wood's Point and at Walhalla in Victoria. 
A second process pneumatolysis, is due to the action of super- 
heateq Water or steam associated with boric and fluoric acids, 
Of Sometimes phosphoric acid. In this case the change is 
More intense; the felspars are altered to kaolinite (china- 
clay), topaz; and tourmaline; and tin, brought up as vapour 
of stannic chloride or stannic fluoride is deposited as tin 
Oxide, cassiterite. This process produces tin ore and others, 
3nd also micy, phosphate, and pegmatite. } 
A third type of deposition is by alkaline solutions which 
dissolve silica and metallic sulphides, and re-deposit them, 
"ith quartz 46 the common veinstone, as the solutions cool 
2thigher levels, This process has formed most of the primary 
odes, ang in them the metals are usually present as sulphides. 
A fourth kind of deposition is by chloride solutions, which 
at the Nevada Hot Springs carry iron, arsenic, antimony, 
30d mercury, These metals may be deposited as sulphides 
or oxides. “Small veins of hematite are formed around 
volcanic steam vents by the decomposition of iron chloride 
by Steam, 
Lobe Srrycryrg AND DeprH oF ORE FormarioN-——The 
Structure of Joes depends upon the spaces in which they are 
formed. Aq solutions rising through fissures are cooled on 
*PProaching the surface they deposit material as successive 
layers or crusts on each wall of the fissure (cf. p. 11). The 
Rature of the layers varies during the formation of the vein. 
The temperature and composition of the water probably 
varies Spasmodically, with renewed earth-movements or in- 
‘rusions, ang these changes affect the layers of these crusti- 
fled lodes (Posepny. Tv. Amer. 1.M.E., xxiii, 1804, p. 207). 
In most lodes the metallic ores are derived from below ; the 
Yeinstones may be largely contributed from the rocks on 
‘he sides, as water percolating through them brings with it 
silica, carbonate of lime, and barium oxide derived from 
felspar, and they are deposited as quartz, calcite, and barite 
(BaSQ,).