ZIRCONIFEROUS SANDSTONE NEAR .ASHLAND, VIRGINIA 273 



but on heating to redness they become colorless. Under the microscope 

 individual crystals are pink or j^ellow, but much the largest number are 

 colorless. The crj^stals in most specimens are very much worn, but the 

 crystals in the specimens from the prospect hole northwest of Mr. Shel- 

 don's house show beautiful crystal form. Though nearly all of the zircon 

 is undoubtedly worn, the wear in general may be in part apparent only, as 

 small zircon crystals formed in place very commonly have outlines that do 

 not show good faces or angles. The difference in the amount of wear of 

 the particles which were caught on a 100-mesh sieve and of those which 

 passed through a 150-mesh sieve is striking (see PI. I, figs. 1 and 2). The 

 greater mass of the larger crystals small as they are seems sufficient to cause 

 much more fracturing from the force of impact when throivn around by 

 waves and currents. 



ASSOCIATED MINERALS. 



Associated with the zircon are quartz and a variety of heavy minerals, 

 including garnet (?), ilmenite, rutile, staurolite, cyanite, and an isotropic 

 green mineral which has not been definitely determined but which may be 

 pleonaste or hercynite. Occasional feldspar and pyrite were noted in several 

 thin sections of the rock. As stated above, these are all cemented with 

 limonite, possibly in part siliceous. 



Ilmenite is the most abundant mineral in the rich pieces and its grains 

 are of about the same size as those of zircon. The quartz and cyanite 

 grains are generally several times as large. In places the fine-grained zir- 

 con and ilmenite surround quartz pebbles an inch long with the other dimen- 

 sions somewhat smaller. 



No magnetite- was found in the material. 



MICROSCOPICAL PETROGRAPHY. 



The petrography of the rock is simple, but the general character of the 

 minerals and their relations to one another and to the cement are more 

 definitely established by microscopic than by megascopic study. Con- 

 sidered as to mineral composition the ten thin sections of the rock studied 

 may be divided into two groups, (1) zircon-ilmenite sandstone and (2) 

 quartz sandstone. The rounding of the ilmenite and zircon grains is 

 pronounced, but the quartz sand is remarkably angular* (see Pis. I and 



I 



* This is in accordance with the investigation of Mackie on the rounding of sand 



grains, who observed that grains of zircon were rounded more readily than those of 

 quartz, due possibly to the greater density of the zircon. See Macliie, Wm., Trans. 

 Edinburgh Geol. Soc, 1897, vol. vii, pp. 298-311. 



