4 UNIVEESITY OF VIEGINIA PUBLICATIONS 



other rocks are sometimes observed in the granite about the border portions 

 of the stocks, and basic segregations^ (schlieren) are common. 



(7) Mineralogieall}^ unlike materials of marked variation in composi- 

 tion and texture, are observed penetrating the granites in places as veins and 

 dikes, some more frequentl}' than others. These include true quartz veins, 

 pegmatite, aplite, and granite dikes of normal composition and texture, 

 among the acid kinds; and dikes of basic igneous rocks, chiefly diabase, 

 among the basic kinds. 



(8) Three sets of joints intersect the granites, a vertical set, a diagonal 

 set, and a horizontal set. The vertical set is usuallj' mors strongly developed 

 than the diagonal, and in some of the granite-masses both sets occur. As 

 regards direction of strike these lie mostly in the northeast and northwest 

 quadrants. Subsequent movement in the granite-masses along the joints, 

 resulting in some cases in faulting, is indicated in the frequent develofiment 

 of slicken-sides on the joint surfaces. Joints which are either curved or 

 approach horizontality. being generally parallel with the granite surface, 

 are of frequent strong development and, in most cases, are probably due to 

 variations of temperature, daily and seasonal. In other cases, notably in the 

 granite domes like Stone Mountain and the numerous smaller ones near 

 Lithonia, Georgia, the outer parts of the granite are in a condition of com- 

 pressive strain, which results in slow exfoliation and the development of 

 joints that run approximately parallel to the surface. 



MIXEEAL COMPOSITIOX. 



Feldspar, quartz, and biotite, in the order named, are the dominant 

 minerals in these rocks throughout the region. The total feldspar varies 

 greatly in amount, but ranges between 43.48 per cent. (Maryland) and 

 81.06 per cent. (Georgia), with an average for the entire region of 64.43 

 per cent, of the total rock mass. Averaged for the individual states, a nearly 

 progressive increase in feldspar from north to south (Maryland, 54.53 per 

 cent.; Virginia, 65.21 per cent.; North Carolina, 61.33 per cent.; South 

 Carolina, 66.84 per cent.; and Georgia, 74.17 per cent.) is indicated, the 

 northernmost state, Maryland, showing the minimnm. and the southernmost 

 state, Georgia, the maximum amount. 



The feldspar composition is a mixture of potassic (orthoclase and 

 microcline, the latter subject to much variation in amount is sometimes in 



^Very seldom are the schlieren of acid character, consisting wholly or nearly so of 

 quartz and feldspar. 



