48 
Till'] GHANI) GASfON DhSTKICT. 
escarpments exjDOsing the edges of the strata before lie became aware of it. 
It is most noticeable in the Trias, and in the secpiel this will be more fully 
discussed. The explanation of this attenuation of the strata towards the 
east is as follows. 
It is a common fact that the greatest thickness of a group of strata is 
usually found near the shore-lines of the mainlands from which their mate- 
rials came. As we recede from these ancient shore-lines we generally find 
that the strata diminish in thickness, at first quite rapidly, but afterwards 
more slowly The materials deposited near the shores are, in many cases, 
of coarser texture than those deposited at a distance from them. This is 
not always true of every distinct bed, but if we consider any group of strata 
with many members we shall usually find it true of the groiqD as a whole. 
In the case of the Mesozoic strata of the terraces, they are remnants of beds 
deposited in a sea or bay, the shore-line of which lay to the westward and 
northwestward. The position of this shore-line, no doubt, varied during 
the Mesozoic periods, now advancing and now receding; but in general 
terms its mean position appears to have been nearly along what is now the 
boundary of the Basin Province. The Great Basin was then dry land, un- 
dergoing denudation, and its detritus was washed down on this side into the 
sea, where the Mesozoic strata of the Plateau Province accumulated. The 
position of this ancient shore-line in the sierra country south of the literal 
Basin and west of the Grand Canon district we do not as yet know ; the 
presumed location not being explored as yet. This attenuation of the 
strata and their relation to the shore-line of the mainland, from which they 
were m great part at least derived, is another important factor which must 
be kept in mind in the course of the discussion. 
WATBESHBUS OB THB TEEEACBS. 
It will be well to bestow also a glance at the distribution of the more 
important drainage channels. The western portion of the terrace is drained 
by the branches of the Virgen River. Upon the Colob heads the northern 
tork of the Virgen, sometimes called the Mu-kiin-tu-weap, sometimes Little 
