526 PRINCIPLES OF STRATIGRAPHY 



The lower part of the tuff consists of a finer grained mass, in 

 which the round mass of felt-Hke glass fibers and wisps contains 

 angular fragments of feldspar, quartz, biotite, and hornblende in 

 greater abundance than in the upper part. Balls of hard green 

 shale occur in the tuff and these and channel-filling of contempo- 

 raneous origin point to a fluviatile deposition of the material. 



In the Bridger of the same region occurs a white tuff ranging 

 in thickness from 25 to 75 feet which carries frequent lignitic beds 

 varying in thickness from six inches to a foot. No clay, sand or 

 gravel occurs as in tuffs of fluviatile origin, and it is believed that 

 this bed is a lacustrine deposit. In regions where the tuff bed is 

 absent it is replaced by arkose beds with clay pellets and apparently 

 wind-blown pyroclastics, the whole traversed by tubules suggestive 

 of root canals, and representing the terrestrial part of the accumu- 

 lation. 



In the Oligocenic of this region pyroclastic material is more 

 pronounced. In the erosion troughs formed at the close of the 

 Eocenic volcanic mud flows accumulated, beginning with a fine- 

 grained buff-colored tuffaceous shale, in which was found a skull 

 of Titanotlicrium heloceras. This was followed by mud flows carry- 

 ing large angular or rounded blocks of hornblende andesite, up to 

 3 or 4 feet in diameter and consisting of ash, pumice, lapilli and 

 pebbles and cobbles of pre-Tertiary quartz, granite, and gneiss, 

 forming an agglomerate 46 feet in thickness near Wagon-bed 

 spring. Its distribution was controlled by the preexisting valleys 

 and so it is absent in many localities. Repeated flows seem to be 

 indicated by what appear like channels of erosion in some of the 

 older flows, filled with andesite cobbles embedded in gray ash. 

 The volcanic eruption seems to have continued throughout the 

 lower Oligocenic, showering the surrounding country with ash and 

 dust. This in some localities accumulated to a thickness of over 

 500 feet. Locally this ash is quite unconsolidated, but, as a rule, it 

 shows calcareous cementation and it may pass upward into a 

 tuffaceous limestone. Angular fragments of more or less devitrified 

 pumice, and sharp splinters of isotropic glass, make up more than 

 fifty per cent, of the soft ash. The rest consists of angular frag- 

 ments of plagioclase, orthoclase resembling microcline, hornblende, 

 biotite and some quartz besides some other minerals. The quartz 

 and microcline are believed to have been intermingled wind-blown 

 material, from the gneiss debris which forms alluvial fans elsewhere 

 in this region. Tuffs of volcanoes near or in the sea will commonly 

 enclose remains of marine organisms. Examples of these have 



