46 WILLIAM J. MILLER 



chiefly the outer portions." According to Harker/ "the pegma- 

 tite may in any case be regarded as representing the residual 

 'mother Hquor' at the end of the process of crystaUization." 

 Pirsson^ states that pegmatite dikes form "in the aheady sohdified 

 parts of the igneous mass." In an important paper on the origin 

 of pegmatite Crosby and Fuller^ say: "Pegmatite is the end 

 product or final crystallization of the original magma." Brogger^ 

 considers certain Norwegian pegmatites as representing the last 

 stage of the consolidation of the granite magma. Such state- 

 ments as the preceding may hold for many of the pegmatites and 

 silexites of the world, but the writer's conclusions (Nos. 4 and 5 

 above) regarding the pegmatites and silexites of the Lyon Moun- 

 tain district are notably different, that is to say both pegmatite 

 and silexite began to develop while the parent magma was still 

 in a relatively highly fluid state. 



It is also commonly stated or assumed that pegmatites and 

 silexites always developed in the form of dikes. But from the 

 evidence above presented for the Lyon Mountain district, it is 

 concluded that many of the pegmatite and silexite masses, espe- 

 cially those of earlier origin, developed as magmatic segregation 

 products because they formed while the inclosing granite was still 

 magmatic with a considerable degree of fluidity, and it is difficult 

 if not impossible to understand how pegmatite and silexite in the 

 form of true dikes could have intruded such a magma. 



For the comparatively few districts in which silexites have 

 been carefully studied it is commonly held that where pegmatite 

 and silexite have both developed from a granite magma the 

 silexite solidified last. As Iddings^ says: "The association of these 

 quartz rocks with pegmatite is such that in many instances they 

 appear to be portions of the magma segregated from the rest and 

 the last to crystalhze." Spurr^ reaches the same conclusion after 



^ A. Harker, The Natural History of the Igneous Rocks (1909), p. 296. 



^ L. V. Pirsson, Rocks and Rock Minerals (1909), p. 175. 



3 W. Crosby and M. Fuller, Amer. Geol., XIX (1897), 165. 



" W. C. Brogger, Zeit. fur Kryst., XVI (1890). 



5 J. P. Iddings, Igneous Rocks, II (1913), 28. 



^ J, E. Spurr, U.S. Geol. Surv., Prof. Paper 55 (1906), p. 115. 



