TIN: ORIGIN OF M1NKRA1 - 249 



veloped crystalline specimens are formed. In such regions hydrated 

 minerals as the zeolites, kaolinite, etc., are dehydrated, forming 

 fcl<lp:irs; silicic acid will replace carbonic acid in limestones; and 

 large areas of limestone have been almost entirely replaced by such 

 silicates as scapolite, pyroxenes, micas, tourmaline, and feldspars. 

 ( M her characteristic minerals of contact metamorphism are epidote, 

 garnets, vesuvianite, spinels, wernerite, andalusite, corundum, 

 apatite, biotite, phlogopite, and all those minerals so common in 

 the granular limestones. Practically there is no sharp line to be 

 drawn separating the two processes of thermal or contact metamor- 

 phism and pneumatolysis ; one begins where the other leaves off, 

 and the two are so intimately related in the formation of some 

 contact minerals as to be quite impossible of separation. 



The volatile matter given off by hot intruded magmas may pos- 

 sibly be exemplified by a study of the gases escaping from active 

 volcanoes, though the difference here would be that of a great de- 

 crease of pressure. With the relief of all pressure, only those com- 

 pounds would exist which are stable at atmospheric pressures, 

 compounds possibly quite different from those which are in solu- 

 tion and which are stable under high pressures and where the 

 volatile gases do not escape freely. The common gases emitted 

 from volcanoes, solfataras, and fumaroles are hydrogen sulphide, 

 sulphur dioxide, carbon dioxide, hydrochloric acid and volatile 

 chlorides and fluorides, steam, nitrogen, and other gases in much 

 smaller quantities. Many minerals are formed as sublimates by 

 the direct condensation and interaction of these gases. They are 

 usually simple in their molecular structure, in contrast to those 

 formed at depths and under pressure. Sulphur is usually present, 

 a product formed by the interaction of SC>2 and HaS, also sulphates 

 and chlorides, as NaCl, PbCl 2 , and where the temperature is 

 high, the chlorides are decomposed, forming oxides as melaconite 

 (CuO), cuprite, magnetite, and hematite; all these minerals are 

 known in the lavas around Vesuvius. 



Hot Solutions. After the cooling of an injected magma has 

 progressed to such a degree that it is possible for the water to exist 

 as such or the temperature has fallen below 365, the critical tem- 

 perature of water, then many minerals are formed or deposited 

 from the water solutions. Water under pressure dissolves many 

 compounds with ease which at the surface or under normal temper- 

 ature and pressure are but slightly soluble or are considered 

 to be insoluble. The solubility of many substances is greatly 



