ZIRCONIFEROUS SANDSTONE NEAR ASHLAND, VIRGINIA 



283 



tion of the zircon favas confirmed the secondary origin of the zircon oxide 

 material. The results of the two varieties of zircon favas analyzed may be 

 expressed as follows: 



Analyses of zircon favas from Brazil {Hussak and Reitinger). 



Commenting on these results the authors say : 



This seems to confirm the supposition already advanced by Hussak that these 

 favas are a product of the decomposition of the very numerous and large zircon crys- 

 tals found in the disintegrated augite syenite of the before-mentioned Serra, although 

 up to this time such a complete decomposition of zircon had not been observed. 



They state, however, that it would be much simpler to consider the 

 zircon favas as a decomposition product of zircon silicates, such as eucolitej 

 eudialyte, lavenite, rosenbuschite, and wohlerite, but that these minerals 

 are not known to have been identified in the zircon-rich syenite of the 

 region. 



Trueman* notes that when igneous rocks containing zircon break down 

 the zircon becomes lusterless and if subjected to wear from erosion the 

 crystals frequently become rounded. In referring to the work by True- 

 man, the Winchellf brothers remark that the available data indicate that 

 when zircon-bearing rocks are anamorphosed the zircon crystals on account 

 of small size and durability are modified but slightly. 



MODE OF OCCURRENCE. 



General statement. 



Classified genetically zircon forms as a product of (1) igneous rocks- 

 crystallization from magma, (2) pegmatite' dikes, (3) occasionally as a 

 product of contact metamorphism, and (4) dynamo-regional metamorphism. 



* Trueman, J. D., Journ. ofGeoL, 1912, vol. xx, pp. 244-257; see also Derby, O,. A., 

 Proc. Rochester Acad. Sci., 1891, vol. 1, pp. 198-203. 



t Winchell, H. V. and A. N., Economic Geology, 1912, vol. vii, pp. 292, 294. 



