Peneplains of the Ozark Highland. — Hershey. 33 
into undulating plains three to five or more miles in width, 
and separated by the ridges which constitute the remnants of 
the main Tertiary peneplain. One of these small peneplains, 
extending westward from Fort Smith, is of particular interest, 
for its surface is sheeted with the gravelly alluvium of the La- 
fayette formation. This fixes the age of the completion of the 
basin valleys as Lafayette, and the plane of their floor 
throughout the Ozark region may be designated the Lafayette 
base-level. In the vicinity of Fort Smith it is about 100 feet 
lower than the main Tertiary peneplain, and over all of south- 
central Arkansas and east-central Indian Territory it main- 
tains a level 75 to 100 feet below the earlier base-level. 
Between the Arkansas river and the Boston mountain, 
there are, among the hills, certain depressed areas which seem 
to represent the Lafayette base-level at a level about seventy- 
five feet below the main Tertiary peneplain as the latter is 
fixed by the general upland surface. In this region the phys- 
iographic features are obscured because of the abnormal south- 
ward slope of the country which has especially favored post- 
Lafayette erosion. But in northern Arkansas, on the Ozark 
plateau, we find the basin valleys well defined. All the prin- 
cipal streams flow in valleys which are duplex in character, 
being composed of a broad upper trough, beneath the floor of 
which is trenched a narrow canyon valley. Along that portion 
of the White river which passes through Missouri there is a 
basin three to five miles in width, trenched through the Lower 
Carboniferous cherty limestones and well down into the 
dolomytes of the Ozark series. Its floor is everywhere dis- 
sected by the canyon valleys of Ozarkian age. but the main 
hill-tops seem to represent a base-level of erosion at a level 
between 200 and 300 feet beneath the main Tertiary peneplain. 
Passing up the tributary valleys, this base-level is represented 
by persistent rock-terraces along the valley sides, some of 
which spread out into benches of sufficient width to be occu- 
pied by farms. They are especially noticeable on the War 
Eagle fork of White river in Arkansas, and the James river 
and its tributaries in Missouri. In this region, the rock-ter- 
race always occurs at about the same level relative to the main 
Tertiary peneplain, namely, nearly 300 feet below it. 
In the extremelv broken and even truly mountainous 
