FAYALITE IN CERTAIN IGNEOUS ROCKS 557 



occurrence of fluorite in the various members of this series is but 

 another expression, in addition to that of the prevalence of the fayahte, 

 of the probable common magmatic source of the various phases of 

 rocks here referred to as belonging to the same series. 



The idea has prevailed, to some extent at least, that fayalite, on 

 account of its extremely basic character and its association with 

 quartz in certain rocks, and with spherulites and lithophysae, prob- 

 ably had only an aqu^o-igneous origin in rocks rather than ordinary 

 igneous origin. In this connection it may be well to call attention 

 to the various occurrences of fayalite previously noted. 



It has often been observed in crystalline slags. The first natural 

 occurrence of fayalite observed^ was in 1839 in nodules in volcanic 

 rocks on the island of Fayal (whence the name is derived), one of 

 the islands of the Azores. In the Alourne Mountains of Ireland it 

 occurs,^ filling drusy cavities in granite. Professor J. P. Iddings,' 

 has described its occurrence with tridymite in spherulites and litho- 

 physae in the obsidian of Obsidian Cliff, Yellowstone National 

 Park; and Iddings and Penfield^ have described its occurrence in 

 the recent obsidian flows of the Lipari Islands. It also occurs in 

 large crystals, apparently in "vugs," in a soda granite at Cheyenne 

 Mountain, Colorado,^ and in a pegmatite dike in hornblende granite 

 at Rockport, Mass.^ In the latter place, as described by Penfield 

 and Forbes, it occurs as a lenticular shell of varying thickness, twelve 

 to sixteen inches in diameter, filled on the inside with loose earthy 

 material and enveloped by a layer of magnetite one inch thick. It 

 is a fact worthy of note that this Rockport granite is a phase of the 

 alkali- rich rock of Essex County.'^ 



It is also found in cavities in the lava of Vesuvius erupted in 1631, 



^ Poggendorl's Annalen, Vol. I.I, p. 160. 



2 Delesse, Bulletins de la Societe geologique de France, Vol. X (1853), p. 571. 



3 J. P. Iddings, Seventh Annual Report, U. S. Geological Survey (1885), pp. 270, 

 279. 



4 J. P. Iddings and S. I.. Penfiei.d, American Journal 0} Science, Vol. XL 

 (1890), p. 75. 



s W. E. Hidden and J. B. Macintosh, ibid.. Vol. X'LI (1891), p. 439, and Vol. 

 XXIX (1885), p. 250. 



6 S. L. Penfield and E. H. Foebes, ihid.. Vol. LI (1890), p. 129. 

 7H. S. Washington, Journal or Geology, Vol. VII (1899), p. 466. 



