286 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [VolYi. 



caused thin sections to be made from them, and has furnished the fol- 

 lowing account of their microscopical structure : 



TUFA FEOM FLORISSANT. 



The method and scheme of classification employed here is that briefly sketched in 

 the Bulletin of the Museum of Comparative Zoology (vol. v, pp. 275-287). By this sys- 

 tem only do we think that the inclosed fragments could be named, for they contain 

 so few crystals that in most cases the base is the principal thing upon which the 

 decision must rest. 



A. — The finer deposit just above the shai>es. 



A medium-grained gray tufa, containing crystals and fragments of feldspar, augite, 

 &c., cemented by a fine earthy groundmass. 



In the thin section it is seen to be an epitome of the volcanic rocks of the Cordilleras. 

 The groundmass holds fragments of basalt, andesite, trachyte, and rhyohte, with de- 

 tached minerals derived from them. 



The basaltic fragments have in part a dense globulitic base porphyriticaUy hold- 

 ing ledge-formed plagioclase crystals and a few augite granules. Some of the basalt 

 is quite coarsely crystallized, approaching the doleritic type. Olivine was observed in 

 some of the fragments, but it is largely altered to a reddish-brown serpentine. Mag- 

 netite is abundant. In many of the fragments the groundmass has decomposed to a 

 reddish-brown mass, which is untranspareut and holds clear crystals of plagioclase. 

 The basaltic fragments have suffered more from alteration and decomposition than 

 any others in the tufa. 



Of andesite, both varieties pointed out by us (1. c, p. 280) occur in this tufa. The 

 first, which is nearest the basalt in composition, has a brown glass as its base, filled 

 with microlites. This base holds minute rectangular and oblong crystals of feldspar. 

 Large microlites of augite and grains of magnetite were seen. Fragments of this are 

 common, and are clear and unaltered. The second variety of andesite was seen to 

 have a dense gray micro-felsitic base, holding ledge-formed feldspars and magnetite 

 grains. Some contained the reddish brown fibers of the destroyed hornblende. Frag- 

 ments of this variety of andesite are quite abundant. 



The trachyte has a light gray, felty, and glassy base, some fragments showing be- 

 sides this only faint traces of polarization caused by incipient feldspars. Other frag- 

 ments show minute, well-formed crystals that appear to be sanidin. Grains of mag- 

 netite occur scattered through the base. This is also quite abundant, and it, as well 

 as the basalt and andesite, surpasses the rhyolite in amount. 



The rhyolite occurs in the form of a more or less clear glass, often cellular. The 

 cells are often drawn out in the direction of the original flow, forming a fibrous struc- 

 ture, which when of a grayish or reddish brown color resembles woody fiber. Some of 

 the fragments contain elliptical cells, and a few shards of water-clear glass free from 

 inclusions were seen. 



Many crystals entire or broken are scattered throughout the groundmass of the 

 tufa. These crystals belong to plagioclase, sanidin, olivine, magnetite, augite, and 

 quartz. But little quartz was observed; one crystal contained trichites and vapor 

 cavities. The trichites are the same as those commonly seen in the quartz of granite, 

 but this appears to have been derived from the lava. The feldspar contains inclu- 

 sions of base, glass, and microlites, and through these the rock from which the feld- 

 spar was derived can often be told. The augites have the characters of andesitic 

 augite. A little palagonite and one crystal of microcline were seen. 



The groundmass of the tufa is composed of comminuted and decomposed material 

 derived from the lavas before described. In the groundmass trachytic and rhyolitic 

 material appears to predominate. This specimen was chosen for description, as it best 

 represented the general characters of the tufas. 



