764 A TREATISE ON METAMORPHISM. 



have been stretched. Most rocks and minerals when stretched quickly pass 

 their elastic limit, rupture takes place, and the dissevered parts are drawn 

 away from each other, often without deformation of the separated masses. 

 The elongations of rocks and portions of rocks are in most cases explained 

 not by pulling or stretching, but by compression or mashing in a direc- 

 tion transverse to that of the elongation, but this process does produce 

 tensile stresses, which often dissever the mineral particles. But the use of 

 the word "stretching" in reference to such rocks implies that tensile stress 

 is the dominant force. The term "stretching" should be dropped, except 

 in those instances where it is shown that tensile stress has been the primary 

 cause of deformation. 



CHANGES IN CHEMICAL COMPOSITION. 



If injection be excluded, changes in the chemical composition of the 

 rocks during metamorphism in the zone of anamorphism are not nearly so 

 great as they are in the zone of katamorphism. It has been explained that 

 in the latter zone, both in the belt of weathering (see Chapter VI, pp. 507-518) 

 and in the belt of cementation (see Chapter VII, pp. 655-656), the chemical 

 composition of the rocks may be very greatly modified. Indeed, the 

 changes of chemical composition are most profound in the belt of weather- 

 ing, and are very important in the belt of cementation. The great changes 

 in these belts are due to rapid and continuous circulation of water, by 

 means of which large quantities of material are transported from one place 

 to another. It has been explained that in the zone of anamorphism the 

 amount of water present is very small and its circulation exceedingly slow. 

 It follows that the water is not capable of transporting large quantities of 

 material considerable distances and thus making great changes in the chem- 

 ical composition of the rocks through extensive areas. In this we have the 

 explanation of the relative constancy of the chemical composition of the 

 equivalent iinmetamorphosed and metamorphosed rocks of the zone of 

 anamorphism. While chemical changes do not greatly modify the average 

 composition of masses of rocks, it has been explained (pp. 682-685) that 

 there is important migration of material for short distances, and therefore 

 profound interior readjustment of the elements in different mineral combi- 

 nations. 



Excluding injection, the chief chemical changes applying to the rock 

 masses as wholes are those of dehydration and decarbonation. Deoxidation 



