Granites of Cecil County, in North-eastern Maryland. 101 



IV. 



The Staurolitic Mica-Schist, and the Isolation 

 of the Rarer Constituents of the Cecil 

 County Granites. 



In the preceding chapters, the petrographical description 'of 

 the Rowlandville and Port Deposit areas has been given. In 

 the first part of the present chapter the microscopical charac- 

 ter of the staurolitic mica-schist which divides these two dis- 

 tricts will be described, while in the second part the results of 

 a study of the heavier minerals separated from the decom- 

 posed granites in various portions of the whole area will be 

 presented. 



r: — PETROGRAPHICAL DESCRIPTION OF THE STAURO- 

 LITIC MICA-SCHIST. 



The field relations of the mica-schist, which occurs a mile 

 south of Liberty Grove, have been discussed on page 66 of 

 the present paper. It is a finely-crumpled rock, composed 

 mainly of quartz and mica. A cleavage cuts across the folds, 

 and in many cases it is quite prominent. These cleavage 

 lines are slightly sinuous through a faulting of the mica laminae 

 which lie in the folds. The variety of cleavage so well shown 

 in this schist is that described by Heim* as ausweichungs- 

 clivage, which is translated by Bonney as "strain-slip-cleav- 

 age," or simply " slip-cleavage," by Dale."*" 



The essential minerals of this schist are mica and quartz. 

 The mica is of two kinds, muscovite and biotite, which are 

 about equal in amount. It occurs in narrow plates, which lie 

 in the folds of the schist. In the process of the folding, 

 these mica crystals have been bent, crumpled, and broken, and 

 in many places a slight faulting was produced, resulting in 

 the slip-cleavage. The quartz occurs as a mosaic of interlock- 



* Mechanismus der Gebirgsbildung-, Vol IT, p. 53. 

 f Amer. Jour. Science, Vol. XI.III p. 317, 1892. 



