208 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
bling away of the fine white sand grains brings out the frost-work 
pattern in a way that is at once pleasing and grotesque. The mysteri- 
ous name Colob is well suited to it. 
The Cretaceous and Tertiary formations are of little importance in the 
region in which we worked; for only on the eastern flank of the Pine 
Valley mountains did we see them exposed in entirety, and the remote- 
ness of this part of the region from the Hurricane fault line, together 
with the evident simplicity of structure of the range turned our atten- 
tion away from this part of the rock series. On Colob the light 
brownish Cretaceous sandstones, limestones, and shales occur, and coal 
beds are associated with them. A small patch of Cretaceous is exposed 
in the canyon of Ash creek not far below Toquerville, but in general the 
remnants of both the Cretaceous and the Tertiary on the downthrown 
western side of the fault are concealed beneath lavas. Of the Tertiary 
beds, which are mostly weak shales and limestones, one member, a 
coarse quartz-pebble conglomerate, is prominent here, in that it furnishes 
quantities of quartz cobbles and pebbles which make up a noticeable part 
of the recent gravels of the district. 
Geographical Conditions under which the Strata were 
deposited. 
The geographical history of the Plateau province during the long 
period from the Devonian era to the end of the Eocene has been well 
described by earlier observers, who state that it is characterized by 
deposition under slowly changing conditions which were uniform over 
a large area. The best account of the region is that of Dutton, of whose 
conclusions a brief summary will be given. After that a few points will 
be discussed where further study suggests that during Triassic and 
Jurassic times land conditions may have lasted longer than has been 
supposed. 
DevoNIAN AND CarBonireRous Deposition. During the Devonian 
era or perhaps earlier, at the end of the second long period of erosion 
of which evidence is found in the walls of the Grand canyon, the sea 
advanced upon an early Paleozoic or Archzan surface of small relief. 
After the Tonto sandstones and shales had been deposited upon this, the 
Lower Carboniferous or Red Wall strata of quartzitic sandstone were 
laid down in gradually deepening water which in time became deep 
enough for the deposition of the heavy Upper Red Wall limestone. 
