JOINT SYSTEMS 



539 



must follow that the massive pegmatites which characterize it must have 

 formed after the period of deformation which induced the banded or 

 schistose structure in the gneiss. Again, this series of pegmatites must 

 have formed prior to the periods of intrusion of the light gray and the 

 dark blue granites, as shown in fig- 



ure 1. 



Joint Systems 



GENERAL CHARACTER 



Figure 7.— Pegmatite cutting across the Schis- 

 tosity of the Granite-gneiss. 



Middendorf quarry, west of Manchester. 



The Virginia granites are inter- 

 sected by three systems of joints — a 

 vertical set, a diagonal set, and a 

 horizontal set. These may be widely 

 spaced or closely spaced. Usually 

 the spacing is sufficiently wide to ad- 

 mit of dimension stone being quar- 

 ried. The vertical set of joints is 

 usually more strongly developed than the diagonal, and in some of the 

 granite-masses both sets occur. 



Measurements of the strike of the joint-planes made in the quarries can 

 be summarized as follows: Two sets of joints whose planes lie in the 

 northeast and northwest quadrants respectively and compose the major 

 jointing, and two minor sets striking east-west and north-south. Strike 

 of the joint-planes in the northeast and northwest quadrants shows the 

 limits of variation to be north 5° east or west to north 80° east or west. 

 Only a few of the planes strike east-west and north-south. 



The inclined joints are less abundant than the vertical ones, and they 

 dip at angles varying from 20 to 82 degrees. The dips are toward the 

 northeast, east, and southeast, northwest and southwest On plate 69, 

 figure 2, and plate 70, figures 1 and 2, are shown the two systems of 

 vertical and inclined joints. Some movement in the granite masses since 

 the formation of the joints is indicated in the development of slickensides 

 on the joint surfaces. Polished and striated surfaces are fairly 

 abundant. 



HORIZONTAL JOINTS 



Joints which approximate horizontality in position are strongly devel- 

 oped in the granites of Virginia and of the southern States in general. 

 Careful observation over the southern region by the writer has developed 

 two characteristics of this set of joints: 



XLIX — Bull. Gbol. Soc. Am., Vol. 17, 1905 



