ORIGIN OF THE PEGMATITES OF MAINE 315 



dated with the orthoclase (or microcline) in graphic granites as in 

 normal granites' is usually albite or oligoclase. 



Both analyses and microscopic studies show that most graphic 

 granites are mixtures of three minerals: (i) quartz, (2) orthoclase 

 or microcline, and (3) a member of the isomorphous series of plagio- 

 clase feldspars. It should be pointed out moreover that if water 

 or other gases were present, as it is almost certain they were, they 

 formed additional components whose amount the analyses do not 

 reveal but whose influence upon the proportions of the other constit- 

 uents may have been great. If graphic granites crystallized from 

 magmas of eutectic proportions, these were therefore eutectics of at 

 least four components. The above series of analyses, though suggesting 

 that the proportions between the constituents of graphic granites are 

 controlled by some laws, can hardly be regarded as proving their eutectic 

 origin. The theoretical value of such analyses, in elucidating the laws 

 governing rock solutions, is impaired by the fact that they take no 

 account of the gaseous components of the magmas. 



Vogt^ states that in many instances, especially when developed 

 on a microscopic scale, the graphic intergrowths represent the last 

 portions of the magma to crystallize. This fact he cites as in harmony 

 with the conception that they represent eutectic residues. While 

 this may be the true relation in some cases, in other cases the graphic 

 granite was unquestionably not the last crystallization from the 

 magma. In the Fisher feldspar quarry in Topsham, for example, 

 where large masses of graphic granite pass gradually and irregularly 

 into large areas of pure quartz and feldspar, the tests of Wright and 

 Larsen (see p. 310) have shown that the quartz of the graphic inter- 

 growths crystallized above 575° C, whereas the quartz of the large 

 pure areas crystallized below 575°. The latter was therefore the later 

 crystallization. The gem and cavity-bearing portions of the Maine 

 pegmatites in almost every instance grade gradually into normal 

 pegmatite containing abundant graphic granite. From the presence 

 of cavities and of the rare minerals, from the general field relations, 

 and from the fact that the quartz of the pockets and the gem-bearing 

 portions wherever tested is of the low-temperature variety, there can 



1 See Clarke, "The Data of Geochemistry," Bull. U.S. Geol. Survey No. Jjo, 369. 



2 Op.cit., 118. 



