276 



UNIVERSITY OF VIRGINIA PUBLICATIONS 



Zircon is an almost constant minor accessory mineral in the crystalline 

 rocks, especially granites and gneisses, of this old shore and its extension 

 westward, and in places it occurs in large masses. Thin sections of granites 

 occurring immediately west of Ashland and at other places in the Piedmont 

 Plateau of Virginia show the presence of zircon, chiefly as inclusions in the 

 quartz and feldspar. Near Goulden post-office, 10 to 15 miles southwest of 

 the Ashland area, pieces of zircon 3 inches in diameter weathered out of 

 pegmatite dikes have been noted on the surface. Massive zircon •v\athout 

 crystal outline, measuring 4 by 6 inches, has been observed in the pegmatites 

 of Amelia County, Virginia. Similar dikes occur in the gneiss-granite 

 complex of the Piedmont Plateau, forming the old shore-line which extends 

 entirely across Virginia from Maryland into North Carolina, roughly coin- 

 ciding with the meridian of 77° 30". The zircon in the sandstone was not 

 derived, however, from the pegmatites in which it occurs in comparatively 

 large masses, but from the granites and gneisses which carry it in innumer- 



bearin^ sa/idstoi 



^ FVe-Cambrian Arani"has andineJsses* * / ' ' 



Fig. 2. GeneraliziED East-West Section Across the Fall-Line near Ash- 

 land, Virginia. 

 Illustrating the occurrence of zircon-bearing sandstone. 



able very small crystals. It seems probable that similar zircon-rich sand- 

 stones may occur at numerous points along this old shore-line. Many zir- 

 con-bearing deposits may be covered bj' later sediments and some may have 

 been removed by erosion, but it is probable that others, which may be 

 richer or poorer, will be discovered along the contact of the granite and 

 gneiss of the Piedmont Plateau with the overlying sediments of the Coastal 

 Plain. 



It is probable that some magnetite was present with the ilmenite, and 

 glauconite is abundant at places in the Calvert formation. The altera- 

 tion of either of these minerals might produce limonite, which forms the 

 cementing material. An occasional pyrite grain was observed in one or 

 two thin sections, and some hand specimens of the rock exhibit cavities 

 which suggest the removal by decay of some previously existing mineral. 

 From microscopic study of thin sections of the rock, it seems more probable, 



