BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



VOL. 11, PP. 475-502, PL. 50 JULY 31, 1900 



CAMBRO-SILURIAN LIMONITE ORES OF PENNSYLVANIA 



BY T. C. HOPKINS 



{Presented before the Society December 30, 1899) 



CONTENTS 



Page 



Introduction : 475 



Geological position of the ores 476 



Description of the ores 477 



Associated minerals and classification . . '. 477 



Nodular ore 477 



Pipe ore , 477 



Brecciated ore 479 



Flake or sheet ore 479 



Fragmental ore 479 



Yellow ocher 480 



Chemical composition , 480 



Associated minerals , 483 



Associated rocks , 483 



White clay 484 



Mode of occurrence 484 



Original source of the iron 489 



Mode of accumulation of the iron 495 



Summary 498 



Literature on the limonite ores 499 



Introduction 



The object of this paper is to explain the genesis of the limonite ores 

 of the limestone valleys in central and eastern Pennsylvania, to show 

 the original sources of the iron, the modes of its accumulation, and to 

 account for its frequent occurrence along with more or less extensive 

 bodies of white and parti-colored clays. The data and conclusions pre- 

 sented are based on field studies of the limonite ores of the limestone 

 areas in the Great valley and in portions of Nittany valley. 



The occurrence of limonite ores in Pennsylvania has been known a 

 great many years ; in fact, they have been employed in the manufacture 

 of iron ever since that industry began in this country. They occur in 



LXVIII— Bull. Geol. Soc. Am., Vol. 11, 1899 (475) 



