MABYLAKD GEOLOGICAL SUEVEY 199 



ents, quartz, potassium and soda-lime feldspars, biotite and accessory 

 minerals. The most noticeable feature of the rock is the secondary 

 gneissic structure, which is brought out by the arrangement of the 

 shreds and flakes of black mica. This arrangement, which is better 

 shown in the ledge and the hand specimen than in a thin section, is 

 seen on examination to be due to small disconnected groups of mica 

 flakes, which lie in approximately parallel lines. These lines are 

 not straight or continuous, but are wavy and the flakes are dissemi- 

 nated or overlapping in such a. way as to produce the well-known len- 

 ticular effect of gneiss. The texture differs from that of true 

 gneisses, however, in showing no banding due to changes in the com- 

 position or coarseness of grain of the rock. 



The color of the rock is a light bluish-gray, which in buildings 

 gives a bright fresh appearance at first and then gradually becomes 

 somewhat darker through an accumulation of the dust and dirt in 

 the atmosphere. Such a darkening of the reck produces a mellowed 

 pleasing effect in structures situated in most of the cities. The 

 roughness of the surface, however, and the abundance of the black 

 mica render the appearance of the older buildings constructed from 

 this rock somewhat sombre, if the atmosphere is strongly charged 

 with dust particles. This is particularly true in cities where soft 

 coal is extensively used without smoke consumers. On the whole 

 the appearance of this rock is unusually pleasing. 



The chemical composition of the Port Deposit granite is shown in 

 the following analysis of a specimen from the McClenahan quarry 

 made by the late Dr. ¥m. Bromwell. Strictly speaking the rock 

 would seem to be a quartz monzonite rather than a true granite since 

 the relative amounts of potash and soda for the feldspars indicate a 

 relative lack of the potassium feldspar. 



Analysis of Port Deposit Granite. 



Si0 2 73.69 



A1 2 3 12 . 89 



F<C0 3 1 . 02 



Feb 2.58 



CaO 3 . 74 



MgO 50 



Na 2 2. SI 



K 2 1 . 48 



H 2 1 . 06 



Total 99.77 



