Granites of Cecil County, in North-eastern Maryland. 95 



The smaller crystal has at the border an undulatory extinction 

 which passes across the epidote rim without any change. In 

 all probability then the relation between the allanite and the 

 epidote is the same as in the granites farther south, where 

 Hobbs and Keyes described similar rims as probably original. 



Among the other accessory constituents, hornblende is rarely 

 present in bright green pleochroic crystals. Magnetite, as is 

 shown by the magnet, on the other hand, is widely distributed 

 in minute grains and large masses, which are often broken 

 and pulled apart. In some of the granites there are numerous 

 red garnets, which are filled with quartz inclusions and have 

 the dodecahedral form, or in a few cases possess a rounded 

 outline. Epidote, though found in the coarse-grained granites, 

 is never as abundant as in the Rowlandville rocks, while in 

 the squeezed granites where the ground mass is very fine, 

 little or no epidote is seen. Chlorite occurs often in large 

 amount, and in some cases is intimately associated with 

 epidote, forming a chlorite epidote aggregate, the latter 

 mineral is in a few specimens arranged in compact groups of 

 needles in the chlorite. Calcite is not often present, and is 

 associated with other decomposition products, as iron hy- 

 droxide and muscovite. 



CHEMICAL COMPOSITION OF THE PORT DEPOSIT 

 GRANITE-GNEISS. 



On account of the economic importance of the granite- 

 gneiss in the Port Deposit quarries, it was thought advisable 

 to determine the relative proportions of the different minerals 

 in it. This has been accomplished in two ways : First, by a 

 mechanical separation of the constituents by means of the 

 Thoulet solution ; and second, by a chemical analysis. The 

 results of the first method are only approximate, because the 

 admixture of secondary minerals w T ith the feldspar caused 

 this mineral to fall through a very considerable range of 

 specific gravity. 



A chemical analysis of the granite-gneiss from McClenahan's 

 quarry was made by Mr. William Bromwell, in the chemical 

 laboratory of the University, with the following result : 



