THE FLORISSANT LAKE BASIN. 23 



These beds of shale vary in color from yellow to dark brown. Above 

 them all lay, as already stated, iVom fifteen to twenty-five decimeters of 

 coarser, more granulated sediments, all but the lower bed broken up and 

 greatly contorted. These reached almost to the summit of the mesa, which 

 was strewn with granitic gravel and a lew pebbles of lava. 



Specimens of these upper irregular beds, and also of the underlying 

 -hales, were submitted to Dr. M. E. Wadsworth, of Cambridge, Massachu- 

 setts, now of Iloughton, Michigan, who caused thin sections to be made 

 from them and has famished the following account of their microscopical 



structure : 



TUFA FROM FLORISSANT. 



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

 the Bulletin of the Museum of Comparative Zoology (vol. 5, pp. '-'75-287). By this 

 xv .Mem only do we think that the inclosed fragments could be named, for they contain 

 so few crystals that in most CMM\S the base is the principal thing upon which the 

 decision must rest. 



A. THE i IXKK DEPOSIT .tr.-i AII<>\ r i in .-HALES. 



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

 augite, etc., cemented by a line earthy groundma-s. 



In the thin section it is seen to beau epitome of the volcanic rocks of the Cordil- 

 leras. The groimdmass holds fragments of basalt, andesite, trachyte, and rhyolite, 

 with detached minerals derived from them. 



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

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

 quite coarsely crystallized, approaching the doleritic type. Ulivinc 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 thegroundmass has decomposed to a 

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

 The basaltic fragments have suffered more from alteration and decomposition than any 

 others in the tufa. 



Of audesite, both varieties pointed ont by us (loc. cit,, p. 280) occur in this tufa. 

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

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

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

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

 have a dense gray micio-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 audesite 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 sauidiu. 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 How, forming a fibrous struct 



