448 RECORDS 



SECTION OF GEOLOGY AND MINERALOGY. 



February 19, 1900. 



Section met at 8:15 P. M., G. F. Kunz presiding. 



The minutes of the last meeting of Section were read and ap- 

 proved. 



The following program was then offered : 



Henry S. Washington, The Magnet Cove Laccolith, 

 Arkansas. 



Summary of Paper. 



The structure of the complex is described and, from the 

 evidence of the form of the area, the relations to the surround- 

 ing shales, the presence of an overlying zone of metamorphosed 

 rock, the arrangement of, and the serial petrographical and 

 chemical characters of the main rock types, with other minor 

 points, the conclusion is drawn that the igneous complex is 

 probably a laccolith, and certainly a unit ; and that the main 

 component abyssal rocks are not due to successive injections, as 

 was suggested by J. F. Williams, but are the result of a dif- 

 ferentiation /;/ situ of the mass of magma. 



The main rock types are described, some new analyses being 

 given, and they are shown to form a regularly graded series of in- 

 teresting rocks, ranging from basic jacupirangite, through ciotite- 

 ijolite, typical ijolite, shonkinitic syenite and leucite-syenite, to 

 foyaite. This serial, and common genetic character is shown 

 both mineralogically and chemically. It is probable that the 

 dikes of tinguaite and nepheline-porphyry are aschistic, while 

 those of monchiquitic rocks are diaschistic. 



The arrangement of the abyssal rocks is shown to differ rad- 

 ically from most other cases of differentiated laccolithic masses 

 and dikes, in that there is progressive increase in acidity toward 

 the periphery. One or two other instances of this are men- 

 tioned, the most closely analogous being the laccolith at Umptek 

 in Kola (Finland). 



An explanation of this is given, based on a process of frac- 

 tional crystallization or freezing of the magma, and the idea ap- 



