ORIGIN OF THE PEGMATITES OF MAINE 303 



except that the pegmatites are probably on the average slightly more 

 quartzose, a conclusion which seems warranted by the frequent transi- 

 tions from pegmatite masses into veins composed largely or wholly 

 of quartz. As is well known, the highly quartzose types show as 

 typical pegmatitic textures as the less quartzose. The difference in 

 the proportions of the principal mineral constituents in the normal 

 granites and the normal pegmatites seems, therefore, insufficient to 

 account for the great difference in their textures. It appears necessary 

 to seek the cause of these contrasts in differences in the proportions 

 of minor constituents or in the presence in the granite or pegmatite 

 magmas of constituents which have since escaped, or which through 

 occlusion are not now visible to the unaided eye, in the derived rocks. 



Influence of minor constituents. — The presence in many pegma- 

 tites of unusual minerals, such as fiuorite and other fluorine-bearing 

 minerals, lithium minerals, boron and phosphorus minerals, and 

 occasionally the rare earth minerals, has led certain geologists' to 

 attribute to some of these substances an important role in the produc- 

 tion of pegmatite textures. It cannot be doubted that when present 

 in magmas such substances have some influence upon the texture of 

 the resulting rock. It has not been demonstrated however that the 

 presence of these unusual constituents is essential to the development 

 of typical pegmatitic textures. In the opinion of the writer their 

 presence is probably not essential. The pegmatites which earliest 

 attracted the attention of American mineralogists and geologists 

 and which have been most often described in the literature were natu- 

 rally those in which unusual minerals were present in especial abun- 

 dance or in perfection of crystal form. Such pegmatites constitute, 

 however, only an exceedingly small proportion of the pegmatite in 

 any district and must be regarded as unusual rather than normal 

 types. The writer is familiar with certain deposits showing typical 

 pegmatitic textures, which have been worked for their feldspar for 

 years with the discovery of few if any of the rarer minerals.^ In the 

 great majority of the pegmatites of Maine unusual minerals are of 



^ Certain French geologists in particular have been advocates of this view. See 

 De Lapparent, Traiie de geologic, 4th ed., 639 (1900); and De Launay, La science 

 geologique, 557-58, 582-83 (1905). 



- The Andrews feldspar quarry in Portland, Conn., the Frost feldspar quarry 

 in Maryland, and the Goldings feldspar quarry in Georgetown, Maine, are examples. 

 See Bull. U.S. Geol. Survey No. 420, 31, 50, and 75. 



