656 



FRANK F. GROUT 



and Auvergnose are well represented. Of the 17 analyses, 17 are 

 persodic, 12 are docalcic, 12 are perfelic, 10 are dosalic. The varia- 

 tion of the others is not uniform. 



Such relationships in composition may be taken as evidence of 

 origin from a common magma, and the differences that exist in the 

 unaltered rocks are therefore to be attributed to some process of 

 differentiation. Alteration is too great for accurate discussion of the 

 subject. Mr. Lane has raised the question of differentiation in a 

 single flow.^ His opportunity to examine extensive drill cores, gave 

 him results that are beyond comparison with any now to be added 

 from a rapid field survey. He found some changes which appeared 

 to be due to an early separation of oligoclase which was lighter than 

 the basic magma, and rose, affecting both composition and texture. 

 Analyses are offered in confirmation, but the high degree of alteration, 

 indicated by 5 per cent water and i per cent carbonic acid, makes the 

 chemical evidence of much less value than the microscopic work. 

 Mr. Lane does not suggest that such action was common in the 



TABLE IX 

 Analyses of Basic Laccoliths of Minnesota 



1. Gabbro, Richmond, Minn. E. M. Pennock, analyst. 



2. Gabbro, Little Falls, Minn. Miss Lillian Nye, student analyst. 



3. Gabbro, Duluth, Minn. Several student analysts obtained results, from 

 1.02 to 103 per cent. The lowest is by G. H. Stone, given below. 



I A. C. Lane, Michigan Geol. Survey, VI, 215. 



