SUMMARY 635 



entiation in 40 cases is shown by diagrams based on alkalies and silica. 

 Magmas do not have to be of any particular composition, to begin with, 

 in order to develop into a differentiated series. The trend or direction 

 of variation is very different in different localities, even when magmas are 

 similar. As noted many years ago, rocks of great similarity may develop 

 from magmas of very different composition as intruded. 



Nearly all the notably alkaline rock-masses have some phase near av- 

 erage nephelite syenite, whether related to gabbro or to granite. It can 

 hardly be assumed that assimilation or syntexis is the dominant factor 

 in producing such a uniform nephelite syenite from such diverse original 

 magmas. 



Primary magma probably approaches average basalt, but varies so 

 widely that it may be truly dioritic or highly alkaline in extreme cases. 



Selected Bibliography or Rock Series 



A. Gradational Series 



Julianehaab. — N. V. Ussing: Geology of the country around Julianehaab, 



Greenland. Copenhagen. 

 S k ye.— Alfred Harker : Tertiary igneous rocks of Skye. Memoirs of the Geo- 

 logical Survey of United Kingdom, 1904. 

 Magnet Cove. — H. S. Washington : The foyalite-ijolite series of Magnet Cove. 



Journal of Geology, volume 9, 1901, pages 645-670. 

 Nordingrd. — J. M. Sobral : Geology of the Nordingra region. Dissertation, 



Upsala, 1913. 

 Haliburton. — F. D. Adams and A. E. Barlow; Geology of the Haliburton and 



Bancroft areas. Canada Department of Mines, Geological Survey Memoir 



6, 1910. 

 Duluth. — Frank F. Grout: A type of igneous differentiation. Journal of 



Geology, volume 26, 1918, pages 626-658. 

 Assynt. — S. J. Shand : On borolanite and its associates in Assynt. Trans- 

 actions of the Geological Society of Edinburgh, volume 9, 1910, pages 376- 



416 (with analyses by Gemmel, pages 417-419). 

 Shonkin Sag, Square Butte, Highaood Peak, Middle Peak. — L. V. Pirsson : 



The igneous rocks of the Highwood Mountains, Montana. U. S. Geological 



Survey Bulletin 237, 1905. 

 Sudbury. — A. P. Coleman: The nickel industry. Canada Department of Mines, 



Mines Branch, number 170. 1913. 

 Essex County.— C. H. Clapp : Geology of the igneous rocks of Essex County, 



Massachusetts. TJ. S. Geological Survey Bulletin 704, 1921. 

 Purcell. — R. A. Daly : Geology of North American Cordillera at the 49th 



parallel. Report of Chief Astronomer (Ottawa), number 6, 1913, pages 



221-255. 

 Haystack Mountain. — W. H. Emmons : Geology of the Haystack stock, Mon 



tana. Journal of Geology, volume 16, 1908, pages 193-229. ■ 



XLH — Bull. Geol. Soc. Am., Vol. 33, 1921 



