Granites of Cecil County, in North-eastern Maryland. 63 



As the Rowlandville granite is followed eastward it retains 

 the normal appearance described above as far as the town of 

 Liberty Grove, although it occasionally carries in addition 

 some secondary calcite. Near this last place the rock be- 

 comes much lighter in color through a decrease in the 

 amount of biotite and an increase in the amount of white 

 feldspar of the plagioclase series. Between these two towns, 

 along the valley of Basin Run, the dark patches form a very 

 characteristic feature, but disappear completely in the vicinity 

 of Liberty Grove. To the south of this village a dyke of 

 Mesozoic diabase was traced for nearly a mile. 



The dark patches referred to above occur in small amount 

 in the Port Deposit quarries and are more abundant in certain 

 granites near Frenchtown, and they merit a special descrip- 

 tion. 



BASIC SEGREGATIONS. 



Oval or irregularly-shaped patches of a darker color and 

 more basic composition than the surrounding rock, have been 

 observed in granite areas the world over. Within the district 

 here under discussion they are sufficiently abundant to de- 

 mand attention, since their origin may throw some light on 

 the nature of the rocks which contain them. The usual type 

 of these dark areas has a distinctly foliated appearance, and 

 is composed mainly of biotite and small masses of white 

 feldspar, but there is considerable variation in composition 

 within short distances. Some are composed of a fine grained 

 aggregate of biotite, quartz and feldspar, or, in other words, a 

 fine granular granite, imbedded in a coarse-grained rock of 

 similar mineralogical composition. Other dark patches occur, 

 which are composed almost wholly of biotite. The line of 

 division between the darker rock and that surrounding it is 

 usually a sharp one. In a few cases the specimens showed a 

 gradual change from a lighter border to a darker interior 

 which was similar to the other dark patches, but its outer 

 margin was not clearly defined on account of this gradation. 

 These dark areas vary both in shape and in size, but the 

 characteristic shape is oval. 



To account for the origin of such dark patches in granite, 

 two hypotheses have been suggested : first, that they are 



