ORIGIN OF PEGMATITES. 727 



little water. On the other hand, the intimate penetration of all parts of the 

 rocks, down to the openings between the mineral particles, by the pegrna- 

 tizing material can be accomplished only by very mobile solutions which 

 are more nearly allied to water than to magma; but such water is very 

 rich in mineral material. Between these extremes are all gradations. 



The aqueo-igneous pegmatites, especially the pegmatitic schists and 

 gneisses, in all their phases may be seen in any of the regions of America 

 in which there are great batholithic intrusions. They are illustrated equally 

 well in the Cordilleras, the Lake Superior region, Canada, and eastern 

 United States from Maine to Alabama. All these regions afford innumer- 

 able perfect illustrations of the process. Even the most extreme pegma- 

 tization is illustrated at many places. One of the best instances known to 

 me is that of Rib and Mosinee hills, central Wisconsin. These hills of 

 quartz rock are surrounded by a great batholith of augite- syenite many 

 kilometers in diameter. Within the quartz rock adjacent to the syenite 

 feldspar has so extensively developed as to change the rock from a nearly 

 pure quartz rock to one in which feldspar is an important constituent. A 

 more extensive and no less perfect illustration of pegmatization, as well as 

 of almost every other phase of the process, is furnished by the Hudson 

 schist in the upper part of Central Park, New York, and even better at New 

 Rochelle, adjacent to Long Island Sound. Williams's accurate description 

 of the phenomena exhibited in Maryland shows this to be an admirable 

 case of pegmatization." 



In summary, peg'matization, when it occurs on a great scale, usually is 

 found in connection with great intrusive masses in which there have been 

 long-continued composite intrusions. No great batholith is the result of a 

 single simple intrusion. The introduction of such masses went on irregu- 

 larly through a very long time. Pegmatitic masses are not the result of a 

 distinct epoch of eruption, but usually are produced in connection with the 

 closing phases of igneous activity. The pegmatites very frequently cut the 

 igneous rocks intruded in an earlier stage of the igneous epoch. After the 

 main masses of igneous rock have crystallized they continue to contract as 

 they cool, and are thereby fractured. This occurs while they are still very 

 hot, and gives ready access to the pegmatizing material. Also the surround- 



o Williams, G. H., The general relations of the granitic rocks in the middle Atlantic Piedmont 

 Plateau: Fifteenth Ann. Kept. U. S. Geol. Survey, 1895, pp. 657-684. 



