Granites of Cecil County, in Xorth-eastern Maryland. 113 



tary origin, and the minerals which have been so extensively 

 developed in it are just those which we might suppose would 

 be developed by the action. 



II. — SUMMARY OF RESULTS. 



The most important general conclusions derived from the 

 foregoing study of the granitic rocks in Cecil County, Mary- 

 land, may be briefly summarized as follows: 



1. The granitic rocks are to be regarded as eruptive 

 masses, which have been variously modified by dynamic 

 agencies. 



2. The division of the area studied into two portions, 

 divided by the belt of staurolitic mica-schist, is shown both 

 on structural, chemical and mineralogical grounds to be well 

 founded. 



a. The rocks near Port Deposit are much more gneissoid 

 and foliated, and hence represent either an older intrusion or 

 a zone of maximum dynamic action. 



b. While the chemical analyses show marked differences 

 in acidity, this is hardly to be relied upon as representing the 

 differences in average bulk composition between the two 

 areas, since the specimen selected from near Rowlandville 

 was unusually dark and rich in biotite. 



c. The alteration of feldspar in the northern or Rowland- 

 ville area is mainly chemical, resulting in a change to epidote, 

 while in the Port Deposit area it is- more mechanical. 



3. The dark patches appear to be basic segregations formed 

 previous to complete solidification rather than inclusions of 

 foreign rocks. 



4. The most striking manifestation of metamorphism is the 

 extensive development of epidote, especially in the Rowland- 

 ville area, in an acid plagioclase. The zonal arrangement of 

 this mineral indicates a zonal development of original iso- 

 morphous layers of the feldspar substance. 



5. The small needles with hexagonal arrangement, in 

 biotite, quartz and feldspar, are to be regarded as epidote. 



6. The staurolitic mica-schist, separating the areas, repre- 

 sents a sedimentary deposit more ancient than the granites, 



