128 WARREN. 



been thought best to put on record the results of the investigation 

 as it stands. 



In recent years J. H. L. Vogt ^ has given us in an elaborate and 

 suggestive paper, an hypothesis regarding the physico-chemical rela- 

 tions of the alkali-feldspars. He has not, so far as the writer is aware, 

 furnished us with a detailed quantitative microscopic and chemical 

 study combined of particular feldspar intergrowths such as is here 

 described. In an admirable paper on the Granite Pegmatites of 

 Tammela in Finland, Eero Makinen ^ has given us a careful chemical 

 and microscopic study of certain perthites from some of the Tammela 

 pegmatites, including a quantitative estimate of the amounts of the 

 two feldspars present in the perthites. Both Vogt's and Makinen's 

 work will be referred to more fully beyond. Aside from Makinen's 

 results there has been, so far as the writer is aware, no quantitative 

 study made of these intergrowths which is at the same time both 

 microscopic and chemical. 



The importance of perthitic feldspars in many members of the 

 granite and syenite families of rocks, and the lack of precise knowl- 

 edge regarding them, is sufficient to indicate the importance of our 

 having further ciuantitative information regarding them and has led 

 the writer to attempt the present investigation. To anyone familiar 

 with the characteristics of perthites and microperthites it will be 

 obvious that data of a very precise character will be difficult to obtain. 

 The coarser textured perthites of the granite pegmatites clearly ofi'er 

 a better chance of yielding quantitative results of greater precision 

 than the finer microperthites of the granitoid rocks and were, there- 

 fore, made the subjects of a first attempt. The results are offered in 

 the belief that they at least furnish a rather close approximation to the 

 truth so far as the granite pegmatite feldspars are concerned, and the 

 hope is entertained that they may, if applied with caution, throw some 

 further light on the mode of origin of perthitic intergrowths in general. 



With regard to the geological relationships of the pegmatites from 

 which the perthites here studied have been taken, little can be stated 

 at the present time. With the exception of the feldspar from Perth, 

 Ontario, and perhaps that from Bedford, Ontario, the pegmatites are 

 connected with an ordinary type of granite, one rather siliceous, low 

 in lime, iron and magnesia, and relatively high in the two alkalies, 



1 Tschermak's Min. u. Pet. Mitt., 24 (1905). 



2 Die Granite Pegmatite von Tammela in Finnland und ihre Minerale, 

 Bull. d. 1. Commission Geologique de Finnlande, 1913. 



