Granites of Cecil County, in North-eastern Maryland. 93 



muscovite. The latter is the more abundant, and is found 

 especially in and near feldspar cracks and along cleavage 

 lines. The alteration to epidote is never as complete as in the 

 Rowlandville granites, and here, as there, the feldspar excites 

 no apparent orienting influence on these small epidote grains. 



The quartz in the Port Deposit gruiiite gneiss is broken 

 into a mosaic of interlocking grains of varying size. In 

 some cases the mosaic is coarse, and then the grains usually 

 show coarse and fine cracks. In other cases the grains are of 

 very small size. This mineral contains numerous inclusions 

 of iron oxide and minute dust-like particles. It also contains 

 small needles like those described in the quartz of the Row- 

 landville granites, but they are here neither so abundant nor 

 so well developed. 



The biotite occurs in the form of a broken aggregate of dark 

 brown pleochroic plates, associated with large fragments of 

 epidote and sphene. In the fresh granite, as exposed in the 

 quarries, these masses do not form continuous bands through 

 the rock, but are arranged in disconnected groups. In the 

 more weathered specimens the biotite forms parallel layers, 

 and the rocks even in thin sections present a distinctly 

 foliated appearance. The biotite bleaches to muscovite and 

 also changes to chlorite. 



The muscovite is usually secondary, and occurs in radiating 

 tufts and irregular masses. In a few of the granites, this 

 mineral occurs in large plates with bent cleavage lamellae, 

 and it is also bordered by fresh biotite. Such original musco- 

 vite, however, is very small in amount. 



In addition to these essential minerals, there occur a num- 

 ber of accessory minerals similar to those found in the Row- 

 landville rocks. Apatite is seen in all the sections, both in 

 cross section, and as small lath-shaped crystals, This mineral 

 is much more abundant in these granites than in the Rowland- 

 ville rocks, while zircon is less so. Sphene of a gray opaque 

 color is also a very abundant constituent. It occurs in 

 lozenge-shaped crystals, irregular masses, and forms leucoxene 

 rims. 



Allanite is present in about one-quarter of the sections in 

 small grains and crystals, and it is always surrounded by a 

 rim of epidote. 



