ZIRCONIFEROUS SANDSTONE NEAR ASHLAND, VIRGINIA 285 



quartz porphyry, trachj'te, phonolite, tephrite, dolerite, diabase, and basalt 

 among volcanic igneous rocks. It has also been reported to be present in 

 phonolite and basalt tuffs.* 



Zircon is most common in the more alkalic rocks, rich in soda, such as 

 the nepheline syenites, phonolites, tinguaites, and tephrites, and is rare in 

 the basic rocks, especially those rich in lime, magnesia, and iron, but has 

 been reported from a number of localities, chiefly foreign, in gabbro, dia- 

 base, basalt, and basalt tuff. It is one of the first minerals to crystallize 

 from the cooling magma, but according to Murgocifit has formed in riebeck- 

 ite rocks during the entire period of consolidation of the magma. 



The minute crystals of zircon usually present in many igneous rocks, 

 chiefly as inclusions in the principal silicate minerals, and which afford 

 distinct evidence of earlier crystallization from the magma were probably 

 formed under high temperature conditions and possibly without the aid of 

 mineralizers. Harkerf, on the other hand, remarks that when zircon occurs 

 in relativelj^ large cr5^stals of later formation and in abundance, it is prob- 

 ably safe to assume the presence of mineralizing agents. 



Zircon has been noted at a number of localities in certain types of 

 igneous rocks in sufficient quantity to give varietal name to the rock. 

 Three occurrences are worthy of note. In the well known augite syenite 

 of Frederiksvaern, Southern Norway, zircon is so abundant as to have 

 given rise to the name zircon syenite. § Closely similar zircon-rich augite 

 and nepheline syenite have been described by Hussak^ and von Sachsen- 

 Coburg from Brazil. In 1882 Wadsworth° announced the occurrence of a 

 zircon-rich syenite about Salem harbor (Marblehead) , Massachusetts, 

 almost identical with the celebrated zircon syenite of Frederiksvaern, 



* For the natural occurrences of zircon see Thiirach, H., Ueber das Vorkommen 

 Mikroskopischer Zirkone und Titan-Mineralien in den Gesteinen, Verhandl. Phys.- 

 Med. Gesell., Wurzburg, 1884, vol. xviii, no. 10, 82 pp.; Brogger, W. C, Zeitschr. 

 Krysl. f. Mill., 1890, vol. xvi.; Michel-Levy, Bull. Soc. Geol. de France, 1883, iii ser., 

 p. 284; Rosenbusch, H., Sulla presenza dello Zircone nelle Rocoie, AM della R. Acca- 

 demia della Scienze di Torino, vol. xvi. 



t Murgoci, C. M., On the Genesis of Riebeckite and Riebeckite Rocks, Amer. 

 Jour. Sci., 1905, vol. xx, p. 137. 



t Harker, A., The Natural History of Igneous Rocks, 1909, p. 291. 



§ Brogger. W. C., Zeitschr. Krysi. f. Min., 1890, vol. xvi. 



If Hussak, E., Mineral Notes from Brazil. Part III. Min. u. Petrog. Mitth., 

 1898-99, vol. xviii, p. 339; see also Recent Occurrence of Zircon in Brazil (Galdas, 

 Province of Minas Geraes) by von Sachsen-Coburg, Min. u. Petrog. Mitth., 1888-89, 

 vol. X, p. 453. 



° Wadsworth, M. E., Proc. Bost. Soc. Nat. Hist., 1882 (1883), vol. xxi, p. 406. 



