BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 4, pp. 305-312 July 31, 1893 



EPIDOTE AS A PRIMARY COMPONENT OF ERUPTIVE ROCKS 



BY CHARLES ROLLIN KEYES 



{Read before the Society December ■'><>. 1892) 

 CONTENTS 



Page 



The epidote-bearmg Rocks 305 



Occurrence and eruptive Origin 305 



Microscopic Characteristics 305 



Comparison of Granites with Granitites 306 



Allanite 306 



The mineralogic Associate of Epidote •"> | ><> 



As a rock-forming Mineral reviewed 306 



Epidote : 308 



Its Abundance in Maryland < rranite 308 



Its microscopic Appearance 308 



Its ( JrystalL igraphy ' 308 



Its Origin 309 



Summary • 311 



The epidote-bearing Rocks. 



Occurrence and eruptive Origin. — Very conclusive evidence has been 

 found recently showing that certain granites of Maryland are eruptive in 

 origin. The granitic masses occur in irregular bosses breaking through 

 gneiss, gabbro and other crystalline rocks. In the central part of the 

 state, at Dorseys Run station, Woodstock, Ilchester and Ellicott City, 

 the acid eruptives are true granitites. granular aggregates, consisting 

 essentially of quartz, feldspar and black mica, with considerable amounts 

 of epidote and allanite as accessory constituents. These rocks vary con- 

 siderably in color, from dark iron-gray to nearly white, according to the 

 percentage of ferro-magnesian silicates they contain. 



Microscopic Characteristics. — Under the microscope thin sections some- 

 times show that the incipient stages of dynamic metamorphism have set 

 in. In other occurrences the granites show little or no signs of mechani- 

 cal deformation. The occurrence of the two prominent accessory min- 



(305) 



