166 GEOLOGY OF THE DENVER BASIX. 



strata, producing very jagged outcrops. Short tree stumps are common 

 in the gravel layers, their roots penetrating the mud beds below. The 

 repetition of this feature shows the conditions of deposition of these beds. 

 The gravel layers ordinarily contain many small pebbles of the light 

 andesite, while darker varieties are in some places almost excluded. The 

 lighter-colored rock is augite-andesite with a very small amount of augite, 

 carrying some hornblende and biotite; the groundmass is often largely 

 crvptocrvstalline. The upper layer of 3 is more nearly a sandstone in 

 composition than the lower beds. It contains very many small pebbles, 

 hut the main substance is ash-like and the rock is thus similar to that 

 of higher layers, designated tuff. 



The strata included under 4 are chiefly clayey in character, but there 

 is really no sharp line at the base, for the upper stratum of 3 differs from 

 the lowest of 4 in being firm and coarse-grained, and the latter passes 

 through admixture of clay and increasing fineness of grain into the beds 

 which are more properly called sandy clays, and so on to epiite pure clay. 

 Upward there is a change in color, the upper clay being dark reddish- 

 brown with spots of lighter color. Pebbles of andesite occur sparingly all 

 through, and a few angular fragments have been found. Tree stumps occur 

 near the middle of this division. The darker clays are full of sand, as is 

 shown by washing them, ami if the sand thus purified be examined under 

 tin- microscope ir will be found to consist of augite, hornblende, biotite, and 

 feldspar. With these minerals are small, round grains representing pebbles 

 and fragments of andesite now almost completely destroyed. A sand of 

 similar composition will be obtained on washing any clayey stratum 

 in the Denver formation. Plant remains are abundant in the division 4, 

 though seldom well preserved. 



The beds of No. 5 of the section are most typical. Deposited upon 

 the hard, brown mottled clay of 4 is a stratum of quite compact sand rock 

 of light-yellow or gray color. This is even-grained and uniform except for 

 pebble-like masses or concretions of a material extremely like the matrix 

 in which they lie, but as a rule somewhat coarser in grain. These masses 

 are sometimes 3 or 4 inches in diameter and in certain layers are thickly 



