704 ABSTRACTS OF PAPERS 



MICROSTRUCTURE OF TITANIFEROUS MAGNETITES* 

 BY JOSEPH T. SINGEWALD, JR. 



(Abstract) 



A study of etched polished surfaces of titaniferous magnetites shows that 

 they consist in part of granular aggregates of magnetite and ilmenite. A 

 microscopic investigation of the magnetite grains reveals further that the latter 

 are not homogeneous, but contain countless minute intergrowths of ilmenite. 

 This paper described the nature of these intergrowths. 



ORIGIN OF GRANITES AS WELL AS METACRYSTALS BY SELECTIVE 

 SOLUTION — A RECANTATION 



BY ALFRED C. LAKE, 



(Abstract) 



Metamorphic rocks having been exposed to increased temperature and pres- 

 sure will not only be recrystallized, but any included moisture will have its 

 solvent power increased. Slowly this solvent power will diminish, after the 

 main ruetainorphic action has taken place, and the pressure is compressive 

 rather than differential as the strata return to the pressure and temperature 

 at the surface. It is at this time that the main inetacrystals, chloritoid, stau- 

 rolite, etcetera, are formed. If a sandstone bed containing a large amount of 

 water, inclosed in slates, like the Berea Grit, were put through the same 

 process, there seems no reason why a very large amount, or the whole of the 

 rock material, should not be dissolved and be reprecipitated, either as the 

 whole mass of strata came nearer the surface or as the dissolved stratum was 

 enabled by fissures to find its way to places more favorable to crystallization. 

 Such rocks are likely to be near a eutectic granite in composition and peg- 

 matitic in texture. The writer did not believe that such rocks existed, but is 

 now inclined to believe that some of the Maine pegmatitic granites may be of 

 this nature. 



Discussion 



Dr. G. P. Merrill remarked that he had seen the rock in outcrops along and 

 in the road running south toward Spruce Point. He had noted the close paral- 

 lelism of the feldspars and also the apparent increase in schistosity parallel 

 with the walls in proximity with its contact with the granite. He questioned 

 if the structural peculiarities were not due to a shearing from crustal move- 

 ments during the cooling and gradual stiffening of the magma. 



PRE-CAMBRIAN STRUCTURE OF THE NORTHERN BLACK HILLS, SOUTH 

 DAKOTA, AND ITS BEARING ON THE HOMESTAKE ORE-BODY * 



BY SIDNEY PAIGE 



Published as pages 293-300 of this volume. 



1 Read with the permission of the Director of the U. S. Bureau of Mines. 



2 Presented at the meeting under the title "Pre-Cambrian structure of the northern 

 Black Htlla as bearing on the origin of the Homestake ore-body." 



