General C. A. McMahon's Address. 467 



some of which prepare the way for those that follow. In some 

 cases the change may be brought about by the removal, in whole or 

 in part, of one or more of the essential constituents of a mineral, 

 whereby the relative proportions and mutual relations of those that 

 remain are altered, as the following examples will show. 



By loss of water limonite passes into hsematite, and opal into 

 crystalline quartz. Dyscrasite, by loss of antimony, passes into 

 native silver, and pyroxene, by the removal of its lime and iron, 

 is changed into talc. Simple oxidation or the absorption of oxygen 

 by a mineral is responsible for another class of changes, as in 

 the conversion of zinc blende into goslarite, and antimony into 

 valentinite. 



The loss of one or more of the ingredients, concurrently with the 

 introduction of one or more new ones, causes many metamorphic 

 changes, as in the conversion of marcasite into magnetite, of 

 witherite into barite, and of azurite into malachite. 



The well-known conversion of a peridotite into serpentine is 

 a case in point. Here, part of the iron and magnesia is removed 

 from the olivine, and water is introduced. A simple process like 

 this, brought about by the percolation of surface waters through 

 an igneous rock, is sufficient to transform considerable areas of rock 

 masses into serpentine, as has been the case in parts of Cornwall. 



Some metamorphic processes are more complex than those alluded 

 to above, but Nature has unlimited time at her disposal, and is able 

 to manufacture potent chemical reagents as her processes proceed. 

 For instance, the sulphides of various metals of common occurrence 

 in rocks, most of which, with the exception of those of the alkaline 

 metals, are insoluble in water, by taking up oxygen pass into 

 sulphates, most of which are soluble in that liquid at the ordinary 

 temperature. These sulphates are readily carried away in solution, 

 and become potent factors of change in rocks through which water 

 charged with these salts flows. Again, carbon dioxide, so abundant 

 in percolating water, decomposes minerals containing lime or alkali, 

 and removes them as soluble carbonates to effect powerful chemical 

 reactions elsewhere. 



I must pass over the subjects of paramorphism and pseudo- 

 morphism, as the limited time at my disposal does not permit me to 

 enter upon these subjects. In the above sketch I have entered 

 upon a discussion of some of the leading principles that seem to 

 me to underlie contact action and metamorphism in the wet way, 

 because I venture to think that, if we really understand these two 

 divisions of our inquiry, it will be unnecessary on the present 

 occasion to enlarge on other branches of our subject. 



Take, for instance, what is commonly called dynamic - meta- 

 morphism. The main factors in this kind of metamorphism are the 

 folding, crumpling, crushing, and shearing of rocks by earth 

 movements, especially during the upheaval of mountains. But 

 these dynamic forces are potent factors in the development of heat. 

 In the case, therefore, of dynamic - metamorphism, as in contact- 

 metamorphism, pressure and heat are the main factors acting in 



