; >14 GEOLOGY OF THE DENVEB BASIN. 



from Archean rocks, is present. Some particles have apparent glass 

 attached to them. 



The cementing substance of this tuff is composed chiefly of a web of 

 minute, clear needles of unknown composition, which act feebly upon 

 polarized light and seem to extinguish parallel to the vertical or prismatic 

 axis. These needles are not visibly attacked by strong hot hydrochloric 

 acid, and are therefore probably not zeolitic in nature. 



As this tuff is composed so largely of clear plagioclase grains the 

 Thoulet solution was used to isolate the predominant mineral for analysis. 

 The result shows that but one feldspar could be separated in any purity, 

 and that one isolated was found to be andesine, with composition as follows 

 (analyst. Dr. W. F. llillebrand): 



Analysis of typical tuff /nun Table Mount/tin, Colorado. 



SiOs 59 - 35 



A"': \ 25.33 



]•>■<>:, > 



i:,o 8-83 



n a o :*• («; 



K o 1.59 



There are small quantities of MgO, HjjO, etc. The tuff, as a whole, 



contains Si0 2 , 58.45; Na 2 0, L.69; K 2 0, 0.66. 



i i i i ICEOUS BEDS IN GENERAL. 



Those beds in which rounded particles appear in very small quantity 

 are naturally of almost identical habit with the type described, and there 

 is nothing new in them excepting the particles of andesitic rocks, usually 

 reddish or brownish in color, and often quite decomposed. The cementing 

 substance seems, in many cases, to be identical with that already referred to. 



Mam beds deserving the name of tuff are coarser-grained than the 

 above type, and consist of fragments and worn particles in about equal 

 parts. In main' of them the eruptive character of the material is obscured 

 by decomposition, and especially by the hydrated iron oxide to which the 

 beds owe their distinctive yellow ishdirown color. 



The cementing material of various tulfaceous beds has been found to 

 be zeolitic, as might be expected, and from frequent occurrence of heuland- 

 ite in many places it seems probable that this species is more commonly 



