GREATER PROBLEMS OF PHYSICAL GEOLOGY. 
55 
presented itself over the western mountain region of the 
United States. One of the most striking cases is the Plateau 
Country. This great region, nearly 100 000 square miles in 
area, lying in the adjacent parts of Colorado, Utah, New 
Mexico, and Arizona, discloses from 8 000 to 12 000 feet of 
mesozoic and cenozoic strata. Here the proof is abundant 
that the surface of the strata was throughout that vast stretch 
of time never more than a few feet from sea level. Again 
and again it emerged from the water a little way, only to be 
submerged. At many horizons grew forests which are now 
represented by those abundant and beautiful fossil woods 
which of late have become celebrated. In the cretaceous we 
find many seams and seamlets of coal or carbonaceous shale; 
but they are included between sandstones wdiich are cross- 
bedded and ripple-marked, or between shales and limestones 
which abound in the remains of marine mollusca. Here the 
evidence seems conclusive that the whole subsidence went on 
at about the same rate as the surface was built up by deposi¬ 
tion. In short, it may be laid down as a general rule that 
where great bodies of sediment have been deposited over ex¬ 
tensive areas their deposition has been accompanied by a 
subsidence of the whole mass. 
The second class of facts is even more instructive, and 
stands in a reciprocal relation to those just mentioned. 
Wherever broad mountain platforms occur and have been 
subjected to great erosion the loss of altitude by degradation 
is made good by a rise of the platform. In the western por¬ 
tion of the United States there occur mountain ranges situ¬ 
ated upon broad and lofty platforms from 20 to 60 miles 
wide and from 50 to 200 miles in length. Some of these 
platforms contain several mountain ridges. All of them 
have been enormously eroded, and if the matter removed 
from them could be replaced it would suffice to build them 
to heights of eight or ten miles; yet it is incredible that 
these mountains were ever much loftier than now, and may 
never have been so lofty. The flanks of these platforms, 
with the upturned edges of the strata reposing against them 
