Granites of Cecil County, in North-eastern Maryland. 97 



upon the massive character of the rock, while in the Port 

 Deposit area the structural changes are much more pro- 

 nounced than the mineralogical. 



The maximum dynamic action took place near Port De- 

 posit, where it was sufficient to bring about a more or less 

 complete recrystallization of the constituents and to superin- 

 duce a decided parallel structure. Thus, the biotite was not 

 only recrystallized but rearranged under the influence of 

 shearing, so as to produce the foliation which is the especial 

 feature of this granite-gneiss. The evidence of mechanical 

 action upon the constituent minerals, in the way of optical 

 disturbance, bending and breaking of the grains, peripheral 

 granulation, and the like, is very much more distinct than in 

 the granites further north, and seem to be, in a way, inversely 

 proportional to the epidotization of the feldspar. From a 

 careful study of this rock, and a comparison with material 

 from other regions where eruptive granites have been clearly 

 shown to be changed by subsequent dynamic action into 

 granite-gneisses, the conclusion that the foliation of the Port 

 Deposit rock is a secondary feature seems to be abundantly 

 warranted. 



The secondary structure thus developed belongs to that 

 type of gneissic arrangement which the Germans have called 

 flasrig. The structure of the Port Deposit granite-gneiss 

 seems'to be intermediate between Xauman's types of kornig- 

 flasriger-Gneiss and flasriger-Gneiss* This term has no exact 

 English equivalent, but it may be translated as lenticular. 

 The arrangement, which is better shown macroscopically in 

 the ledge and hand specimen than in the thin section, thorgh 

 it is clearly distinguished under the microscope, is caused by 

 a series of compressed biotitic lamellae, more or less interlock- 

 ing, and arranged in wavy layers. The small mica groups are 1 

 disconnected, but they lie in approximately parallel lines. 

 This foliation stands nearly vertical, and its direction is north- 

 east, thus conforming both to the main structural features of 

 the region and to the direction of the principal joints. 



Two other structures are seen under the microscope, 

 namely, the micro-porphyritic and the micro-pegmatitic. 

 Though the first of these is of rare occurrence, the second is 



*Nauman Geoguosie, Vol. I, p. 546, 1858. 



