348 PHYSICAL GEOLOGY 



in rocks affected by intrusions. (2) The heat developed by pressure 

 will be discussed in a later paragraph. 



Moisture. — When moisture is present in considerable quantity, 

 as is the case with sedimentary rocks, the effects of heat and pressure 

 in producing metamorphic changes are greatly increased. This is 

 true because highly heated water, especially if alkalies are present, 

 readily dissolves minerals which would otherwise be insoluble, and 

 from the solution the same minerals or new ones may be formed. 

 Water also takes part in the chemical composition of some minerals, 

 such as mica, and is therefore necessary for their formation. The 

 recrystallized and newly formed minerals are usually arranged with 

 their longer axes at right angles to the pressure (p. 349), and are more 

 stable under the new conditions than if they had not been changed. 

 The potency of moisture is shown by the fact that rock which re- 

 quires a temperature of 2500 F. for melting when dry, becomes pasty 

 at 750 F. when water is present. The effect of gaseous emanations 

 in producing metamorphism is sometimes of the greatest importance. 



Pressure. — Simple downward pressure, such as that which results 

 from the weight of overlying rocks, has some metamorphic effect and 

 also tends to consolidate the sediments by bringing the grains closer 

 together. But when the crust is under enormous lateral pressure, 

 as a result of the contraction of the earth, the strata are folded, crushed, 

 and mashed together, and metamorphism takes place. In this 

 way pebbles, fossils, and crystals are flattened and elongated, or 

 broken into fragments. By this agent alone the texture of rocks 

 can be changed, but it is in combination with heat and moisture 

 that the production of new minerals and the formation of highly 

 metamorphic rock is brought about. 



The importance of lateral pressure in the production of regional metamorphism 

 has been questioned by certain French geologists 1 who believe that it is brought about 

 by heat, moisture, and vertical pressure without the aid of lateral pressure; that the 

 sediments in the lower parts of thick geosynclines are actually fused by heat from the 

 interior of the earth, and upon cooling become igneous rocks, capable in their turn of 

 metamorphosing by contact the rocks which surround them. They hold that this 

 is by far the most important element in the process of metamorphism and that dynamic 

 action can deform but cannot transform rock; i.e., it is not competent by itself to pro- 

 duce metamorphic changes. This theory has been generally abandoned by Ameri- 

 can geologists and in fact by many eminent French geologists. 



How the Parallel Arrangement of Minerals is Produced. — The 

 conditions favorable for the production of metamorphism having 

 1 Haug, — Traits de Geologie, pp. 172-191 ; 234-235. 



