ORB DEPOSITS 7 



(2) Solfataras. This includes all waters in which sulphur is 

 present either as a sulphide, as hydrogen sulphide or sulphur diox- 

 i< li -. and sulphurous acid. Hydrogen sulphide plays an important 

 part in ascending solutions. Its source is in the decomposition of 

 pvrito or gypsum. Its presence is the direct cause of a great 

 number of minerals. The sulphide of the metals as a rule is the 

 most important contributor to the source of the metal for com- 

 merce. Gold, iron and tin are the most noteworthy exceptions 

 and others will be considered in the discussion of the respective 

 metals. 



(3) Thermal Waters. These include hot springs and geysers. 

 The temperature may arise from meteoric waters percolating 

 through hot volcanic areas or magmatic waters slowly working 

 their way toward the surface. More than one-half of the 

 known elements have been found in solution in mineral waters. 

 The elements were dissolved in the mineral waters themselves. 

 They are found deposited in iron and manganese formations 

 around mineral springs. They are dissolved and precipitated 

 by the action of the mineral waters upon foreign bodies. 



Origin of Cavities. The form that ore bodies assume is 

 such as to prove that they were often deposited in cavities and 



Fio. 3. Ideal section through a limestone region showing caves left by the 

 removal of the rock. 



fissures in the rocks. These cavities were formed by numerous 

 causes. 



(1) By acidulated waters dissolving the soluble constituents 

 of rock masses as shown in Fig. 3, and by the mechanical action 

 of water, that is, waters wearing away material by the force of 

 impact thereby enlarging fissures already formed. 



(2) Cavities are produced by dolomitization. Magnesium 

 carbonate is readily taken into solution by carbonated waters 

 under pressure. If in the downward transference of meteoric 



