312 GEOLOGY OF THE DENVER BASIN. 



from various rocks, the latter strata being called sandstones. The tuff is a 

 sediment of rapid formation, the material being chiefly derived from some 

 single source; and as in this case the constituent particles are largely 

 crystallized minerals, the whole may be supposed to approximate to the 

 composition of the rock or magma from which the particles come. The 

 purity or homogeneity of the tuff probably corresponds closely to the 

 rapidity of its accumulation, for while a sandstone of identical composition 

 is conceivable, the larg'er time involved in its deposition makes the 

 admixture of material from other sources and of different character highly 

 probable. 



Transition stages would naturally arise as the period of tuff formation 

 was succeeded by one in which a greater and greater admixture of sand 

 or clay entered into the sediments. Such a change ma)' be seen in the 

 beds on South Table Mountain at the northeastern extremity, where the 

 most typical tuff is succeeded by sediments in which clay and sand enter 

 until they are predominant. Then comes again a stratum of coarse grain 

 but with decided admixture of sand particles different in character from 

 those of the tuff, and often this is a line conglomerate containing small 

 pebbles of various rocks along with rounded grains of a rock resembling 

 the tuff in general composition. 



Rocks which are pure tuft's in the above sense are rare in the Denver 

 series. There are, however, numerous transition deposits which partake 

 of the characteristics of both tuff and sandstone, as they will now lie 

 described for the more typical cases. 



THE TYPICAL TUFF. 



occurrence. — The purest tuff known occurs at a horizon from 35 to 60 

 feet below the basaltic capping sheet of Table Mountain. It seems to 

 have a somewhat local development, for although found all along the 

 northern and eastern slopes of South Table Mountain, and locally 

 identified on North 'Fable .Mountain, it is not found on the slopes of 

 Green Mountain, where its peculiar character would seem to render 

 outcrops probable. In the section at the northeastern point of South 

 Table Mountain, and also at the southeastern extremity of the same 



