ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN. 83 1 



(so-called) base that has so puzzled lithologists in the study of 

 the felsites. The rhyolites of all volcanic rocks preeminently 

 show lamination produced by flowing, a fact which is doubtless 

 due to their being so siliceous. This structure and their devitri- 

 fication enables us to trace a direct connection between the rhyo- 

 lites 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 dis- 

 tinct 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. 1. 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 atten- 

 attention 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. 



T 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, Bui. 17 U. S. G. S. 1885, p. 40. 



