558 OGILVIE 



occurred. The chemical system used is the artificial one of 

 Osann. From the metamorphic standpoint three divisions are 

 made according to the depth at which the alteration took place, 

 certain minerals being taken as indications of each zone. In its 

 main lines Grubenmann's system is built upon precisely the 

 foundation which is needed for a natural classification of the 

 metamorphic rocks, but in its details it seems to be open to two 

 objections. One of these is the artificial character of the chem- 

 ical basis; the other, the practical difficulty of recognizing the 

 rock types formed in the respective zones, since the several types 

 of mineral sometimes occur in the same rock. This is especially 

 the case with the middle and lower zones and it appears to be 

 practically an impossibility to know from the minerals only to 

 which of them a given rock type can be referred. The Maine 

 rocks do not fit into either of the zones as defined by Gruben- 

 mann, though their resemblances are more nearly with the 

 lowermost. The prefix "kata" is attached to this by him, 

 which seems unfortunate as that has already been used by Van 

 Hise in the word " katamorphism " as a designation of the highest 

 zone. 



It yet remains for future workers to determine whether it is 

 possible to build up a system that shall accurately measure 

 pressure, heat and stress by means of the minerals formed. In 

 the present state of knowledge it appears better to attempt no 

 such subdivision, but to designate by the prefix "meta" any 

 kind of metamorphism exclusive of weathering, and to apply 

 this prefix to the subrang name of the quantitative system. 



The conception of metamorphism here entertained is that of 

 alteration without addition or subtraction of material. Ob- 

 viously rocks injected, cemented, weathered or otherwise chemi- 

 cally changed would not be available for classification in this 

 manner. In the region here discussed there is no reason for 

 suspecting any changes in chemical composition, and it is believed 

 that the quantitative system furnishes the most logical method 

 of regarding them. 



