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ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN.
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(so-called) base that has so puzzled lithologists in the study of the felsites. The rhyolites of all volcanic rocks preëminently show lamination produced by flowing, a fact which is doubtless due to their being so siliceous. This structure and their devitrification enables us to trace a direct connection between the rhyolites and felsites, which are simply the older and more altered rhyolites......One of the best illustrations of this is to be found on Marblehead Neck, Mass., where at least two distinct flows of felsite occur, one cutting the other. They show the fluidal structure so characteristic of rhyolites,—a character that has been mistaken for lines of sedimentation by geologists. While the enclosed crystals of orthoclase have been taken for pebbles......While to the naked eye and under the microscope this rock shows the fluidal structure of a rhyolite, in p. l. it is seen that the base has been completely devitrified, a process that is carried to a great extent in many known modern rhyolites." No other American petrographer has so distinctly advocated the identity of felsites and ancient rhyolites in spite of the fact that many of our felsites illustrate it as unmistakably as do the English felsites. Dr. Irving[1] in his description of the Beaver Bay group of the Keweenaw series repeatedly calls attenattention to the resemblance between the ancient felsites and quartz-porphyries and the modern rhyolites, although he does not express an opinion as to their equivalence. The statement "that the degree of crystallization developed in igneous rocks is mainly dependent upon the conditions of heat and pressure under which the mass has cooled and is independent of geological time" made by Messrs. Hague and Iddings[2] expresses essentially the position of American petrographers on this question.

Apparently in none of the felsites elsewhere described have the varied structures of the modern rhyolite been more perfectly and conspicuously preserved than in the aporhyolites of the South Mountain.

  1. ↑ Opus cit., pp. 312, 313, note 5, p. 436.
  2. ↑ On the Development of Crystallization in the Igneous Rocks of Washoe, Nevada, with Notes on the Geology of the District,Bul. 17 U. S. G. S. 1885, p. 40.
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