ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN. 817 



longulites and trichites of iron oxide. Zirkel figures and 

 describes a similar appearance in the rhyolites of the 40th 

 parallel. 1 He speaks of faint granular lines "which by their 

 fluidal running form a net with a multitude of-meshes of oval 

 shape." The meshes are filled by one of two types of crystalli- 

 zation, the micro-felsitic or the spherulitic. The lines suggested 

 to Zirkel perlitic parting. In the ancient lavas of South Moun- 

 tain the meshes are filled by the micropoikilitic areas or by 

 spherulitic crystallization or by intermediate stages of altera- 

 tion, that is, spherulites more or less broken up into micro- 

 poikilitic areas. In the trichitic spherulites of the modern 

 rhyolites 2 there is an appearance similar to the micropoikilitic 

 mottling, caused by the breaking up of the radiating spherulitic 

 fibers into irregular areas which extinguish differently ; just such 

 an intermediate stage between the spherulitic and a completely 

 micropoikilitic crystallization as has been noted in the ancient 

 volcanics. These observations suggest that the micropoikilitic 

 structure represents recrystallized spherulitic growths when it is 

 not the direct results of infiltration and devitrification. In many 

 cases, the crystallization has undoubtedly never been spherulitic, 

 if however, the micropoikilitic structure has been shown to be 

 subsequent to spherulitic crystallization, that is, to the consolida- 

 tion of the rock in numerous instances in the acid volcanics, 

 selected from widely separated localities in the South Mountain, 

 the presumption favors the secondary origin of the micropoikilitic 

 structure wherever present in these rocks. 



Spherulitic structure. — Two sorts of spherulitic crystallization 

 are present in these rocks. They differ in no essential respect 

 but are unlike in appearance. The most numerous spherulites 

 are also the simplest and smallest. They are colorless micro- 

 scopic spheres, scarcely or not at all perceptible in ordinary light 

 but showing the usual distinct dark cross between nicols. Spheru- 



1 Vol. VI., Geo. Exp. of the 40th parallel, Fig. 1, PL VI., Fig. 1, PI. VIII. 



2 Sections of material from the Rosita Hills, Colorado, and of the Obsidian Cliff, 

 Y. N. P., were kindly loaned the writer for comparative study by Dr. Cross and Pro- 

 fessor Iddings. 



