ZIRCONIFEROUS SANDSTONE NEAR ASHLAND, VIRGINIA 275 



Quartz. 



Quartz is present in every thin section but varies greatly in amount, 

 from occasional grains in the zircon-ilmenite-rich rock to the dominant 

 and vastly the most abundant mineral in the quartz-rich rock. It is like- 

 wise subject to much variation in size and shape of grain. The grains 

 generally range between 0.2 and 1 mm. in diameter, though smaller and 

 larger ones were noted, and in contrast to zircon and ilmenite are mostly 

 angular in outline. Well rounded grains are not numerous. 



The quartz graii\s are of granitic character and some contain abundant 

 liquid and solid inclusions. Manj^ of them show pronounced strain shadows 

 as the result of d5'namic forces to which the original rock from which they 

 were derived was subjected. The quartz grains in the same thin section 

 will usually average larger in size and more angular in outline than the zir- 

 con. The general character of the quartz grains is shown in Plate II, 

 figure 2. 



Cement. 



In hand specimens the cement is a decided reddish-brown color. In 

 thin sections it is opaque and generally brown in reflected light, and occa- 

 sionalh' transparent and deep red in transmitted light. It is sharply differ- 

 entiated from the mineral grains, which are remarkably fresh. No grada- 

 tion from the iron-bearing mineral grains into the cement was observed, 

 such as would be expected if the cement were derived by alteration of those 

 iron-bearing minerals present in the rock. 



GENESIS. 



The zircon and ilmenite concentration evidently represents an old beach 

 segregation along but within the western margin of the Miocene sediments 

 of the Coastal Plain, of probable Calvert age, and is similar to the black- 

 sand beaches of New Jersey, California, Oregon, New Zealand, New South 

 Wales, and numerous other coasts, and to the gold-bearing garnet (so- 

 called "ruby") sands of the beaches at Nome, Alaska (see fig. 2). 



The zircon and other heavy minerals resistant to atmospheric agencies 

 were derived by weathering processes from the crystalline rocks, chiefly 

 granites and gneisses, of the Piedmont Plateau, which extend westward 

 from the Coastal Plain contact. These formed the country rock of the 

 shore, and the zircon and associated minerals derived from them by weath- 

 ering were accumulated bj^ waters near the mouth of a small stream or 

 behind a sheltered point, while the quartz sand was largelj' worn and car- 

 ried away by the currents of the sea. 



