ORIGIN OF PEGMATITES. 725 



are included the results of three somewhat distinct phases of action, which, 

 however, grade into one another; first, pegmatization mainly aqueous; 

 second, pegmatization mainly igneous, and third, aqueo-igneous pegmati- 

 zation. 



The first case is illustrated in the Marquette district of the Lake 

 Superior region," where at certain places in the Michigamme formation 

 there is a great development of feldspar within the interstices of the schists. 

 In the crevices minute pegmatitic veins of quartz and feldspar occur. No 

 adjacent intrusive in this formation has been discovered even by the closest 

 detailed study. In this case it seems clear that pegmatization has taken 

 place during the metamorphism of the rocks, in connection with mechanical 

 action, without the assistance of any extraneous igneous material, and is 

 therefore essentially aqueous. 



The second of these cases is illustrated by the pegmatites described by 

 Brogger in southern Norway, b in . which igneous injection satisfactorily 

 explains all the phenomena. It is to be noticed that in this region the 

 process of pegmatization occurred at a maximum depth of 600 meters, 

 while much of it occurred at a depth vaiying from 30 to 100 meters. 



The third phase of pegmatization, which combines both aqueous and 

 igneous agencies, is far the most extended and important. By it are formed 

 the great class of the rocks which are usually spoken of as pegmatized. 

 This phase may affect massive igneous rocks, massive sedimentary rocks, 

 and their metamorphic equivalents. It occurs in both acid and basic rocks. 

 In all, the effects may be most complex, ranging from the formation of 

 great pegmatite dikes to the development of individual crystals of feldspar, 

 quartz, and other minerals anywhere within the intruded masses. 



But this phase of pegmatization is most extensive and best illustrated 

 by rocks in which there is a gneissic or schistic structure, since, as pointed 

 out on page 710, cleavage furnishes planes of weakness which are readily 

 taken advantage of by the igneous rocks. The relations are best studied 

 where intrusive and intruded rocks are of different colors; for instance, 

 where there is a dark- colored schist intruded by granite. In such rocks 

 will be seen a background of schist. Parallel to the folia are innumerable 



«Van Hiae, C. R., and Bayley, W. S., The Marquette iron-bearing district of Michigan: Mon. 

 U. S. Geol. Survey, vol. 28, 1897, pp. 447-148. 



b Brogger, W. C, Syenitpegmatitgiinge der sudnorwegischen Augit- und Nephelinsyenite: Zeitsch. 

 fiir Kryst., vol. 16, 1890, pp. 215-235. 



