SOME EXAMPLES OF ROCK VARIATION 39 1 



to those here described have been well described by Messrs. 

 Dakyns and Teall from some Scottish plutonic rocks. 1 The field 

 studies have shown the relations of the various members of this 

 series from Crystal Falls to be as follows : The diorite is found 

 cutting the hornblende-gabbro. The gabbro is also found to 

 be cut by a dike of biotite-granite. The relation of this par- 

 ticular dike to the diorite could not be determined ; therefore it 

 has not been described. In one case, however, a dike cutting a 

 gabbro showed biotite-granite as a facies of the diorite. It is 

 probable that the other dikes of biotite-granite occurring in the 

 area are facies of the same widely distributed diorite magma. The 

 hornblende-gabbro is cut by the bronzite-norite and the perido- 

 tite. It is thus evident that the eruption of the hornblende- 

 gabbro was followed by that of a peridotite on the one hand 

 and by a diorite, possibly even a granite, on the other. 



It is a difficult matter to estimate quantitatively the amount 

 of the one or the other rock type present in the Crystal Falls 

 district. We are thus prevented from drawing from the pre- 

 dominance of the one kind or the other the conclusion that 

 those represented in the minority are the results of the differen- 

 tiation of a magma most nearly resembling in its original con- 

 stitution that which predominates. Moreover, since the analyzed 

 rock types were not selected as representatives of the extremes 

 of the process of differentiation, it would not be wise to endeavor 

 to give the mean composition of the parent magma, from the 

 analyses of the differentiation products which have been pre- 

 sented. The main thesis, however, seems to be established that 

 the separation of a magma into the various products described 

 has taken place, as is indicated by the relations in the field, and 

 as has been shown by the microscopical and chemical analyses. 



From the relations described as existing between the various 



The gabbros and norites of the " Cortlandt Series" on the Hudson River near 

 Peekskill, N. Y., by G. H. Williams, Am. Jour. Sci., Vol. XXXV, 1888, pp. 438- 

 448. 



J On the plutonic rocks of Garabal Hill and Meall Breac, by J. R. Dakyns, 

 Esq., M.A., and J. J. II. Teall, Esq., M.A., F.R.S., F.G.S., Q. J. G. S., Vol. XLVIII, 

 1892, pp. 104-121. 



