Granites of Cecil County, in North-eastern Maryland. 87 



their being secondary epidote, which is, in other forms, so 

 very abundant in these Rowlandville granites. 



MUSCOVITIZATION. 



The alteration of feldspar into muscovite is much less fre- 

 quent in the Rowlandville granites than that to epidote ; and 

 when it does occur, it is small in amount. In such cases it 

 presents a marked contrast to the epidote in a number of fea- 

 tures. The mica usually has a definite form, occurring in 

 small tabular crystals, often with ragged edges. While it 

 sometimes agrees with the epidote in being arranged irregu- 

 larly through the feldspar, it usually shows a decided ten- 

 dency to follow the cleavage lines and twinning striae of its 

 host. The reason why certain feldspars alter into muscovite. 

 while others alter into epidote, depends doubtless on differ- 

 ences in chemical composition. The whole process of mus- 

 covitization in the Rowlandville area, however, sinks into 

 insignificance when compared with the extensive develop- 

 ment of epidote. 



Pressure Effects on the Different Minerals. 



The minerals which show the most marked effects of pres- 

 sure are the two essential components, quartz and feldspar. 

 The former is often broken into a coarse mosaic, as repre- 

 sented in Figure 1 ; though in a few cases a large plate of 

 quartz remains intact or slightly fissured, but it always shows 

 undulatory extinction and peripheral granulation. The feld- 

 spar often shows undulatory extinction, microcline borders, 

 cataclastic structure, or peripheral granulation. In a few 

 cases, the feldspar crystals are broken and pushed asunder. 

 Other minerals also show the results of pressure. The 

 biotite is often broken into irregular masses bunched together, 

 in diversely oriented groups, while both the micas show bent 

 and broken cleavage lamellae. The apatite and magnetite 

 crystals are occasionally broken and separated. Such de- 

 formations as are here noted show that the rocks of the 

 Rowlandville area have been subjected to very considerable 

 pressure, which resulted in a shearing action, causing the 



