4o8 REGINALD A. DALY 



the flow having undergone true magmatic splitting under the influence 

 of a maximum rate of coohng at the surface. As a result of the 

 magmatic differentiation the lava crystallized differently near the 

 surface and within the main body of the flow. To petrologists of the 

 present day this view must seem highly improbable, as it involves 

 an impossible speed of molecular diffusion. The alternative view, 

 rejected by Streng, that the andesitic, phenocryst-free surface phase 

 is due to settlement of the olivine and pyroxene, certainly seems more 

 worthy of behef. 



Secrmdly, the hypothesis of fractional crystallization might be 

 tested by a comparison of the analyses of the world's average olivine 

 basalt, augite andesite, and ultra-basic rocks, along with the average 

 analysis of each of the staple phenocrysts in olivine basalt. An 

 approximation to most of these averages has been made possible 

 through Osaun's great compilation of the rock-analyses made between 

 the years 1884 and 1900.^ From his tables an average of all the 

 available typical analyses for each rock-species has been calculated 

 by the present writer. 



In Table I, Column 1, the average composition of t6i typical 

 basalts (largely olivine-bearing), is entered. In Columns 2 to 6 are 

 entered, in order, the average compositions of 11 Hawaiian basalts, 

 17 ohvine diabases, 9 dolerites, 11 melaphyres, and 17 olivine gabbros. 

 In Column 7 is entered the average of the 198 analyses which include 

 all but the olivine gabbros. Since some of the basalts are olivine- 

 free (perhaps through settling out of the phenocryst) it seems prob- 

 able that the addition of the 17 olivine gabbro analyses to the average 

 total would render it more nearly representative of the true world- 

 average than that shown in Column 7. 



Column 8 of Table II indicates the result of averaging all 215 

 analyses given in partial averages in Columns i, 3, 4, 5 and 6, Table I, 

 and hence represents rather closely the mean composition of olivine 

 basalt throughout the world. 



Column 9 gives the result of averaging 33 of the most typical augite 

 andesites in Osann's compilation. Columns 10, 11, and 12 give the 

 similar averages of, respectively, 49 peridotites, 7 limburgites and 3 

 picrites. 



I A. Osann, Beitrage ziir chemischen Petrographie, Part 2, Stuttgart, 1905. 



