2 9 o MATERIALS AND THEIR ARRANGEMENT 



tute metamorphism, in the generally accepted meaning of the term, 

 but all are metamorphic changes, if that term be given its largest 

 meaning. 



Surface vs. deep-seated changes. Near the surface, the action 

 of water commonly tends to the decomposition of rock; but below 

 a few hundred, or at most a few thousand feet, its general effect is 



Fig. 283. Section of a septarian nodule (clay ironstone). About y& natural 

 size. (Geikie.) 



to solidify the rock, for at these depths deposition exceeds solu- 

 tion, and oxidation, carbonation, etc., go on much more slowly than 

 near the surface, or not at all. Oxidized and hydrated sediments 

 may be buried to great depths, and under the pressure and perhaps 

 the high temperature of these depths, deoxidation and dehydration 

 may take place, with resulting diminution of volume. These 

 changes at considerable depth are one phase of metamorphism, 

 even according to the older use of the term. 



Incipient crystallization. A common metamorphic change in 

 sedimentary rock is incipient crystallization. Some limestones and 

 dolomites are made up largely of small crystals, though the mass 

 was originally a calcareous mud or ooze. New crystals also are 



