MARYLAND GEOLOGICAL SURVEY 109 



nected dikes and included bodies of the same rock in the neighbor- 

 hood of the large masses, and contact action upon the mica-gneiss. 

 The gabbro belt is so intimately related to the meta-pyroxenites and 

 meta-peridotites" that it is highly improbable that one mass is intrusive 

 and the other not. There is nothing in the contours and contacts of 

 the gabbro belt that is inconsistent with an intrusive origin. The 

 meta-gabbro dikes in the granite-gneiss are presumably related to 

 the main gabbro body and indicate an intrusive character relative to 

 the granite-gneiss. 



The same reasoning applies, though with less force, to the granite. 



It is true that the contact of the granite-gneiss and gabbro is 

 usually one of gradation. This fact, however, is not inconsistent 

 with the intrusion of the latter into the former, or of both into the 

 mica-gneiss. The contact of granite-gneiss and mica-gneiss is not 

 uncovered. 



Dynamic movements have affected gabbro, granite and mica-gneiss 

 alike. No more movements are found recorded in one formation 

 than in the others. Cleavage dips and joint planes are parallel in 

 these formations. 



This granite has been considered an intrusive body by other inves- 

 tigators in Maryland and the writer finds no proof against that con- 

 clusion while still holding it provisionally in the absence of positive 

 proof in Cecil county. 



If the igneous formations are intrusive in the mica-gneiss they are 

 of a later age and are either pre-Cambrian or Palaeozoic. That they 

 cannot be ascribed to a later era than the Palaeozoic is rendered plain 

 by the fact that they nowhere intrude into the Mesozoic formations 

 which cover them in the north. 



They are for the present involved in the obscurity which surrounds 

 the age of the mica-gneiss. 



The petrographic study of the eruptive rOcks brings up another 

 question in regard to their origin. Are the igneous formations a 

 geologic unit representing a single intrusion or are successive intru- 

 sions represented by the different petrographic types? It is highly 

 probable that the igneous rocks of the county are the differentiation 



