390 GEOLOGY OF THE DENVER BASIN. 



The fire-clays of the Dakota are bedded, delicately laminated, and 

 locally jointed. They are hard, compact, and fine-grained, the purer varie- 

 ties being almost free from grit. The color is either a dark, bluish gray or 

 a lighter shade, passing into a drab. The darker varieties contain a greater 

 amount of carbonaceous matter, which is distributed either uniformly and 

 invisibly through the mass of the clay or in more or less extended patches 

 of lignitic particles on the planes of lamination. The impurities are a fine 

 sand in thin layers, or oxide of iron, the result of the decomposition of 

 minute grains of pyrite. The latter imparts to the clay a characteristic 

 spotted appearance, sufficient to determine its Dakota horizon where other- 

 wise — as in the area of the unconformity at Golden — through proximity 

 to the dark Benton shales, it might be wholly misinterpreted. 



Following is an analysis 1 of an average specimen of Golden fire-clay, 

 obtained from the mines north of the city: 



Per cent. 



SiO a 50.35 



TiOo 80 



Al ; o 3 33.64 



Fe 2 3 75 



MgO Trace. 



Na ; OH 



K a O 40 



H>0 and organic 13. 88 



100. 00 

 Much organic matter. 



The composition of pure kaolinite is: Si0 2 , 4G.30 per cent: AL.< >„ 39.80 

 per cent; H 2 0, 13. 'JO per cent. 



Computed on the basis of the kaolinite composition, the above analysis 

 would become — 



Per ceiit. 



SiO a combined 30. 134 



AI0O3 33.64 



1I ; combined 11. 75 



Ti0 2 80 



SiOj quartz 11.216 



KO 40 



NasO 09 



MgO Tin,, 



Fe 3 0:j 75 



1 1 « > hygroscopic 2. 13 



100. 00 



1 Analysis by Dr. W. F. Hillebraud, IT. S. Geological Survey. ' 



