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THE JOURNAL OF GEOLOGY.

have exactly the same optical orientation, and must hence extinguish the light between crossed nicols together. Such a structure is termed, according to the particular form it assumes, micropegmatitic or granophyric.

"In the third place a single large crystal of one of the two constituents of the groundmass may be filled with much smaller, irregularly arranged grains or crystals of the other. This would also give the general effect of a finely granular structure, although it is essentially different from either of the others above mentioned."[1]

The same structure was briefly described by Teall in a quartz-felsite from the Cheviot Hills, but without any particular designation being applied to it.[2] Harker also mentions a variety of the same structure as common in the ancient rhyolitic lavas of Wales.[3] Cross described the macro-poikilitic structure in a hornblende-peridotite, from Custer county, Colorado,[4] and the micropoikilitic structure in a rhyolite from Silver Cliff in the same district, although the connection between the two was not mentioned. In speaking of the latter rock, he says of the groundmass:

"There seems to be no isotropic matter, but individual characteristics of form and optical action are lost through the minute size of the grains which overlap and overlie each other in the thinnest attainable sections. This mixture is irregular in many cases, but on others a mottled appearance is produced in that one substance attains a uniform optical orientation in certain areas, but is filled by inclusions of the other substance. No regular intergrowth of the two can be discovered. In some spots it was clearly quartz which was the enveloping mineral."[5]

Brögger has described the groundmass of a quartz-porphyry from the region of Christiana as having a typical poikilitic structure.[6]

In his recent monograph on the Eruptive Rocks of Electric Peak and Sepulchre Mountain in the Yellowstone Park, Iddings describes the micropoikilitic structure in the groundmass of certain dike porphyrites, where he for the first time makes use of exactly this term.[7] In speaking of the Sepulchre Mountain dikes,

  1. ↑ Loc. cit. pp. 367, 368.
  2. ↑ British Petrography, p. 343.London, 1888.
  3. ↑ The Bala Volcanic Series.Cambridge, 1889, pp. 22, 23.
  4. ↑ Proc. Colorado Scientific Society, vol. 2, p. 242.1888.
  5. ↑ Ib., p. 232.
  6. ↑ Zeitsch. für Kryst. u. Min., vol. 16., 1890., p. 46.
  7. ↑ Twelfth Ann. Rept. U. S. Geol. Survey, p. 589.1892.
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