■274 UNIVERSITY OF VIRGINIA PUBLICATIONS 



II) . Both are cemented more or less firmly by oxide of iron, chiefly limon- 

 ite and probably a less hydrous oxide of reddish color, possibly gothite or 

 hematite. 



Of the minerals present in the sandstone, zircon, ilmenite, and quartz 

 are the three most abundant, and are described below in the ord-er named. 

 Occasional grains of an unstriated feldspar were noted in one or two of 

 the quartz-rich sections, and red- to yellowish-brown rutile in partially 

 rounded grains of variable size is sometimes present, always in association 

 with ilmenite. Ferromagnesian minerals are entirely absent. 



Zirco7i. 



In the thin sections the zircon is mostly colorless, though occasional Hght 

 yellow and pinkish crystals were observed, and is readilj^ identified by its 

 high refraction and double refraction. The grains usually show rounded out- 

 line and manj^ of them are nearly spherical. They range from approxi- 

 mately equidimensional to elongate forms, and the angles or corners of 

 those that exhibit squarish to rectangular cross-sections usually show more 

 or less rounding (Pis. I and II). Crystal outline is frequently observed but 

 as a rule it is modified by rounding from wear. Zircon grains separated 

 from the rock and examined under the microscope usually show rounding 

 from wear and rather dull luster (PL I, figs. 1 and 2). The most perfectly 

 rounded grains are apt to exhibit the least luster. The zircon grains 

 average from 2 to 0.3 mm. in diameter. Some of the larger elongated 

 grains measure as much as 1.1 mm. in the direction of elongation. Some 

 grains show cleavage and man}' indeterminable inclusions. 



A fairly noticeable feature in many of the zircon grains is an apparent 

 irregular, thin, cloudy and light-colored peripheral zone or border, which 

 appears isotropic or but feebly double refracting. This border probably 

 represents the pitted surface made by pounding against other fragments 

 and possibly to some alteration from hj'dration. 



Ilmenite. 



Ilmenite is most abundant in the zircon-rich thin sections. It exceeds 

 zircon in amount and is least in quantity in the quartz-rich sections, and 

 almost absent from some. It is remarkably fresh, in grains of about the 

 same size as the zircon grains, probably most of them a fraction larger,* and 

 of irregular though somewhat rounded outline. 



