54 WILLIAM J. MILLER 



Conclusions regarding pegmatites associated with gabbro.— From 

 many occurrences like the examples above described it is concluded 

 that acidic masses ranging from pegmatite rich in potash feldspar 

 and quartz, with or without tourmaline, to pegmatite which is 

 mostly potash feldspar, to pegmatite rich in acidic plagioclase 

 with or without quartz, to basic pegmatite which consists either 

 of basic plagioclase and hornblende or nearly all basic plagioclase, 

 to aplite dikes, and even to nearly pure silica (silexite), developed 

 as satellites of the stocks of normal gabbro. It is also concluded 

 that the development of pegmatite began while the gabbro was still 

 notably fluid, and that it continued until it had almost or completely 

 solidified. Accordingly it seems necessary to regard the earlier- 

 formed pegmatite masses to have developed as segregation masses 

 rather than as true dikes. 



It is possible, as maintained by Grout^ for the Duluth gabbro, 

 that some development of pegmatite may have taken place before 

 the beginning of crystallization of the gabbro magma, but the 

 evidence in the North Creek district is not conclusive. Many of 

 the North Creek pegmatites differ from the Duluth pegmatites 

 in one important respect, namely, that they developed as satellites 

 of the gabbro late in the stages of magma consolidation, some of 

 them probably even after complete solidification. 



Such a profuse development of so many kinds of pegmatites 

 from gabbro bodies is not in agreement with the commonly held 

 view as stated by Weinschenk, who says that pegmatites "are 

 relatively rare with plagioclase rocks," and "the more basic the 

 plutonic rock, the simpler are its satellites."^ 



Grout's work on the origin of the pegmatites of the Duluth 

 gabbro and the writer's work on the origin of the pegmatites of 

 the granites of the Lyon Mountain district and of the gabbro of 

 the North Creek district clearly show that many of the pegmatites 

 of those districts are not, as is so commonly stated for other regions, 

 to be regarded as intrusions into already solidified portions of the 

 magma from which they were derived. 



^ F. F. Grout, Econ. Geol., XIII (1918), 190-92. 



^ E. Weinschenk- Johannsen, Fundamental Principles (1916), p. 142. 



