I 12 



Cincinnati Society of Natural History. 



It is now generally conceded that this cerium-epidote repre- 

 sents a direct crystallization from an original magma, and its 

 presence forms a good proof of eruptive character. 



At a number of localities in this Cecil County area, there 

 occur numerous dark, oval areas in the granite, which all the 

 evidence at hand seems to show are segregations of the more 

 basic constituents of the acid rock. Such a phenomenon 

 must have taken place in the magma while it was in the pro- 

 cess of cooling, and so would point to an original molten 

 state. The structure of these rocks, also, in so far as it has 

 not been entirely remodeled by dynamic action, is that com- 

 monly observed in igneous granites, while the sequence of 

 crystallization in the different minerals also accords with this 

 view. In the main, the structure is hypidiomorphic granular, 

 but this shows a frequent tendency to develop micro-porphy- 

 ritic facies, or inclines to micro-pegmatitic intergrowths of 

 quartz and feldspar. That the structure of some of these 

 granites was more or less miarolitic is shown by the presence 

 of intersticial calcite.* 



3. Geological Mode of Occurrence. — A study of the granite- 

 gabbro contact along the road from Rowlandville to Porter's 

 Bridge seems to show that the granite is the younger rock, 

 but in the Port Deposit region the granite-gneiss seems to be 

 older than the gabbros, as represented in the gabbro-diorite 

 dykes. It may be that the Rowlandville granite is much 

 younger than the Port Deposit granite-gneiss, and the two 

 eruptions were separated by a period of great disturbance, 

 which resulted in the foliation of the Port Deposit granite. 

 The contact above mentioned is that of an undoubted erup- 

 tive rock, the granite penetrating the gabbro in small dykes, 

 and including fragments of the basic rock. There is nothing 

 in the field relations of these rocks which militates against 

 the idea of their being intrusive masses. 



4. Contact Action. — One of the best indications that the 

 granite rocks of Cecil County are of igneous origin is the 

 occurrence of the staurolitic-mica-schist belt between the 

 Rowlandville and Port Deposit areas. There is good evidence 

 for believing that the material of this schist is of sedimen- 



:: Rosenbusch : Die massigen Gesteine, p. 34, 1887. 



