HYDRO- AND ATMOMETAMORPHISM 767 



applied. As already noted, waters carrying magnesium locally alter 

 limestone into dolomites. Sulphuretted springs on the Sinai Penin- 

 sula and at several points along the west coast of the Red Sea alter 

 coral limestones to gypsum, and silica-bearing waters locally alter 

 many reef limestones into siliceous rocks, as in the Upper Devonic 

 coral reef limestones of Grund in Harz and elsewhere. 



The greatest amount of alteration is, however, produced by the 

 highly heated waters, given off by the igneous rock, or accompany- 

 ing the intrusion. Here must be classed all the phenomena of 

 secondary enrichment of the wall rocks of fissures, by the rising 

 waters, though the actual deposits in the fissures themselves are to 

 be classed as hydrogenic rocks. The subject is too special to be 

 pursued here at greater length. The student is referred to the cur- 

 rent works on ore deposits and to \"an Hise's Treatise on Meta- 

 morphism. The work of geysers and hot springs not directly asso- 

 ciated with volcanic intrusions, in so far as it affects the wall rock, 

 must also be classed here. The deposits formed by these agencies, 

 however, are hydrogenic deposits. 



3. Atmometamorphism. Contact with rising vapors and gases, 

 as in solfataras, fumaroles, etc., constitutes a third type of contact 

 metamorphism, to which the term gaseo- or atmometamorphism may 

 be applied. In its broadest sense the weathering of rocks in con- 

 tact with the great gas envelope of the earth, the atmosphere, also 

 belongs here, having the same relation to the intrusion of gases as 

 the surface flow has to the intruded igneous masses. Practically, 

 however, we may confine gaseous metamorphism to the work of hot 

 vapors and gases emitted in connection with volcanic activity and 

 always accompanying pyrometamorphism. Such effects are visible 

 to-day in solfataras and fumaroles where the alteration of the wall 

 rock is proceeding at a rapid rate. 



The high temperature of the vapors and their high content of 

 active chemical agents make their work of alteration much more 

 effective than the work of water solution would be. The work of 

 the gases at very high temperature is called fumarolic, while the 

 work of the gases at lower temperature is solfataric. 



The most important gases active in the fissures of the rock are : 

 water vapor, sulphurous oxide (SO„), chlorine (CI,), hydrochloric 

 acid (HCl), hydrofluoric acid (HFl), hydrosulphuric acid (H^S), 

 sulphuric acid gas (SO3), carbon dioxide (COo), oxygen (Oo) and 

 hydrogen (H,). Nitrogen is of course an abundant but essentially 

 useless gas, and boric acid (H3BO3) is sometimes plentiful. Among 

 the processes going on in atmometamorphism are the common ones 

 of oxidation, hydration and carbonation. The last produces sodium 



