110 THE CRYSTALLINE ROCKS OF CECIL CO-UK TY 



products of a single magma. The problem is one of the time of the 

 differentiation. Was the magma differentiated before or after intru- 

 sion? The types can be explained on the supposition that differenti- 

 ation took place both before and after intrusion. 



The granitic magma of medium acidity was first intruded. 1 In 

 this magma some differentiation took place, thus producing the ap- 

 pearance of gradation along the gabbro contact. Subsequently a 

 more acid magma (meta-rhyolite) filled the fissures in the cooled and 

 contracted granite. The gabbroitic intrusion followed, penetrating 

 into the mica-gneiss, the granite-gneiss and the meta-rhyolite, and 

 representing the basic residual after the separation of the rhyolitic 

 magma. 



Last of all and representing a final differentiation of the magma, 

 there was intruded the iiltrabasic material which gave rise to pyrox- 

 enites and periclotites (serpentine). 



Petrographic study of the igneous types shows that there is a 

 central body of intermediate acidity. Into this body are intruded 

 material both of greater acidity and greater basicity. On either 

 margin of the central mass, with some intermediate types, are succes- 

 sively basic and ultrabasic types. 



The petrographic succession is as follows, passing from the most 

 acid type northward: 



Basic biotite-granite (biotite-qnartz-monzonite), containing dikes of 

 a more acid character (meta-rhyolite). 



Basic hornblende-biotite-granite (hornblende-quartz-monzonite) . 



Qnartz-biotite-hornblende-gabbro. 



Quartz-hornblende-gabbro. 



Hornblende-norite and qnartz-norite. 



]STorite and hypersthene-gabbro. 



Pyroxenite. 



Peridotite. 



1 The granite masses elsewhere through the State have been considered younger 

 than the gabbro (U. S. Geol. Survey, 15th Annual Rep't, p. 738). If this is the case 

 in Cecil county the gabbroitic dikes in the granite cannot be apophyses of the main 

 gabbro mass. The writer has considered them to be such in one case at least, and 

 on this basis the gabbroitic intrusion is held to be subsequent to the invasion of the 

 granitic magma. 



