140 THE CKYSTALLINE ROCKS OF CECIL COUNTY 



The percentages are as follows: 



Si0 2 69.17 



CaO 2.93 



Na 2 5.70 



K 2 20 



Determined by W. F. Hillebrand of the U. S. Geological Survey. 



Albite is a predominating constituent. 



These dikes widen to the southwest and dwindle to the northeast. 



META-GABBEO. 



Some hornblendic dike rocks, in the town and immediate vicinity 

 of Port Deposit have been described on page 97. 



The few microscopic slides that were secured of these show 

 epidote, hornblende, quartz and feldspar as the constituents. The 

 first two predominate. The green hornblende bears the aspect of a 

 secondary product from pyroxene. Epidote is a reactionary product 

 of the f erromagnesian constituent and the feldspar. 



Quartz occurs in a granulated mosaic and in veins which may be 

 secondary. The feldspar shows polysynthetic twinning. It is gran- 

 ular in character and so crushed or altered as not to admit of indu- 

 bitable determination of its species. The equal extinction angles 

 are high and an extinction of 38° was obtained on the basal pinacoid. 

 These results led to the conclusion that considerable basic feldspar 

 was present. 



In the absence of a chemical analysis of these rocks, they are placed 

 somewhat provisionally with the meta-gabbro. 



DIABASE. 



The location and the character of the outcrops of the diabase dikes 

 have been given (pp. 100-101). Of the two dikes, one possesses a 

 greater width and coarser grain than the other. Both are composed of 

 material which, when fresh, is of a dark stone-grey color and a dense 

 texture. When the rock is altered the color changes to green owing 

 to the development of hornblende and epidote. The weathered sur- 



