240 PROCEEDINGS OF THE AMERICAN ACADEMY. 



American geologists that the vast development of granodiorite in the 

 Cordilleras of North and South America should alone give the name a 

 primary place in rock classification, is again justified. The many 

 occurrences of dacite throughout the world represent just so many 

 additional masses of cooled magma which were chemically identical 

 with, or closely related to granodiorite. In volumetric importance, as 

 in mineralogical and chemical individuality, the granodiorite type 

 should rank as of the same order as granite itself. 



Quartz porphyry, liparite, and rhyolite show that essential identity 

 of composition which has long been apparent from more qualitative 

 comparison. 



5. There is little noteworthy chemical difference between the aver- 

 age pre-Gambrian granite and the average granite of later periods. 

 How far the differences in alumina and potash (columns 1, 2, and 3) 

 are due to the relative fewness of analyses of pre-Cambrian types 

 cannot be stated. In spite of any such uncertainties the stability of 

 the chemical type represented by granite throughout geological time 

 is manifest. The explanation of the fact may well be found in Vogt's 

 idea that granite is an "anchi-eutectic," a crystallized mother-liquor, 

 a nearly extreme product of magmatic differentiation. It is possible 

 that some of the older pre-Cambrian granite represents the differentia- 

 tion of primeval magna. For many reasons it seems probable that 

 most, if not all, post-Cambrian granites are differentiates from syntec- 

 tic magma, chiefly composed of primary basaltic magma which has 

 locally redissolved the ancient, acid shell overlying. In such case the 

 splitting of the syntectic would ultimately give an acid differentiate 

 similar to that formed in the primitive time. In general, differentia- 

 tion in batholiths, when well advanced, restores the condition tempo- 

 rarily disturbed by magmatic assimilation. On this (confessedly 

 hypothetical) view one may feel no surprise in noting a fairly steady 

 composition in the granites from the average oldest type to the 

 average youngest. 



Massachusetts Institute op Technology, 

 Boston, January, 1910. 



