1889.] Methods and Models in Gcographie Teaching. 575 
with hills of moderate height ; and thus the very opposite of the 
Caroline plains, where the surface is an upland, with occasional 
valleys. Such an old plain may be seen about the head-waters 
of the Missouri, in eastern Montana; the general surface is 
extremely monotonous, gently rolling, and one roll like the next, 
so that one may easily lose his way in the absence of landmarks. 
But here and there over the plain mesas of considerable elevation 
still remain, the reason for their endurance being seen in the 
layer of hard lava that protects them, and retards their destruc- 
tion, while the rest of the country not thus protected has wasted 
away more rapidly. These lava-caps are old flows from once 
active volcanoes; the lava at the time of eruption undoubtedly 
ran down from its vents to the lowest ground that it could find; 
and yet it now occupies the highest ground, in virtue of its 
obstinate refusal to waste away. Every such lava-cap is an out- 
spoken witness to the greater mass of material over the whole 
country when the eruption took place, and the destruction of this 
greater mass must have progressed through the several stages 
illustrated by the present condition of the Red River plains, 
the Carolina plains, the mountains- of West Virginia, and the hills 
of central Kentucky, before it could have reached a surface of 
faint relief. It requires great faith in the evidence here adduced 
to believe that so stupendous a piece of work has really been 
accomplished. It is well nigh incredible, and the observer on 
the ground is fully justified in doubting it as long as he can; but 
It cannot be doubted when the evidence is once well seized. It 
is by no means unparalleled, and much nearer home we may find 
examples as extraordinary, and as far from easy belief, but as 
necessary to the convictions of the well-ordered geographer. 
Such a plain as that of the upper Missouri may be called a 
base-level plain, because it has been worn down to the controlling 
level of drainage, or to what is called the base-level of the re*gion; 
this being in distinction to a constructional or new plain, whose 
smoothness is due to the short time that its original form has 
been exposed to developing agencies. A base-level plain repre- 
sents the ultimate stage in the sequence of a simple cycle of 
development. 
