220 THE CRYSTAL FALLS IRON-BE AH lis' G DlSTiUCT. 



CLASSIFICATION. 



These rocks just described, from their iniiieralogical composition, if we 

 admit the presence of a vitreous base, would belong with the picrite- 

 porplivrites of Ro.senbusch.^ Tliis designation does not seem, liowever, to 

 he appropriate, as he states" that he uses the term "porphyrite" only for 

 certain textural jjhases of rocks containing lime-soda feldspar. He has 

 evidently extended that definition so as to be able to use it for these 

 jjicrites, considering that the glass possesses the necessary ingredients for 

 the formation of such lime-soda feldspar, provided the conditions under 

 which it cooled had been favorable for the feldspar development. 



The porphyritic texture of these Crystal Falls picrites and the presence 

 of a vitreous base^ show them to be closely related to rocks of effusive char- 

 acter. Those which they most closely resemble among the younger basaltic 

 lavas are the porphyritic forms of the limburgites (magma basalts). 



One of the best-known rocks with which this may be closely compared, 

 as far as association is concerned, is the rock first described by H. Carville 

 Lewis as a saxonite-porphyry,* later called kimberlite. This was described 

 by him as volcanic, and as associated with dolerites and melaphyres. He 

 described it as a basic lava.^ Other occurrences of very closely related basic 

 rocks having a vitreous base have been described from the United States by 

 Diller, Williams, Merrill, Branner and Brackett, Kemp, and Darton and Kemp." 



I Microscopisclie Pbysiographie, by H. Rosenbiisch : 3fl ed., Stuttgart, Vol. II, 1896, p. 1191. 



•^Op. cit., p. 436. 



"Should the vitreous base be considered as not having been present and the rocks be )iut among 

 the peridotites, then they ■vromld correspond very closely to the wehrlite described on p. 254. 



■" Papers and notes, cit., p. 50. 



''The genesis of the diamond, by H. C. Lewis : Science, Vol. VIII, 1886, p. 345. 



On a diamondiferous peridotite and the genesis of the diamond, by H. C. Lewis; Geol. Mag., 

 3d series, A^'ol. IV, 1887, p. 22. 



"Dikes of peridotite cutting the carboniferous rocks of Keutnckj-, by J. S. Diller: Science, 1885, 

 p. 65; Notes on the peridotite of Elliot County, Kentucky, by J. S. Diller: Am. .Tour. Sci., Vol. XXXIl, 

 1886, p. 188 : Bull. U. S. Geol. Survey, No. 38, 1887. 



The serpentine (peridotite) occurring iu the Onondaga salt group, at Syracuse, New York, by 

 G.H.Williams: Am. Jour. Sci., A'ol. XXXIV, 1887, p. 137; Proc. Geol. Soc. Am., Vol. I, 1889, p. 533; 

 Perowskit in serpentin von Syracuse, New York, by G. H. Williams : Neues Jahrb. Vol. II, 1887, p. 263. 



On a peridotite from Little Deer Isle, in Penobscot Bay, Maine, by G. P. Merrill : Proc. U. S. Nat. 

 Mus., 1888, p. 191. 



The peridotite of Pike County, Arkansas, by J. C. Brauner and R.N Brackett: Am. Jour. Sci., 

 Vol. XXXVIII, 1889, p. 50. 



Peridotite dikes iu the Portage sandstone of Ithaca, New York, by J. F. Ke,nip: Am. Jour. Sci., 

 Vol. XLII, 1891,p. 410. 



A newly-discovered dike at Dewitt, near Syracuse, New York, by N. H. Darton and J. F. Kemj) i 

 Am. Jour. Sci., Vol. XLIX. 1895, p. 450. 



I'be rock described by F. L. Ransome as a fourchite should perhaps also be compared with these 



