. Bulletin of the Geological Society of America. Geology. 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
. Bulletin of the Geological Society of America. Geology. 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 posit
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