70 
PALEONTOLOGICAL GEOLOGY 
beneath the oldest Eocene beds as generally recognized, and 
which, according to the paleobotanists, carries a distinctive and 
extensive Tertiary flora. This is a planation plane which seems 
to extend northeastward beyond the Black Hills; and although 
there rather inconspicuous has the Lance formation resting upon 
it. In the Black Hills region, therefore, it is surely a major hia¬ 
tus, perhaps the greatest that we know on the American continent. 
The next great planation level is the Maya plane which is the 
beveled summit of the lofty Mesa de Maya, preserved by old 
basalt flows. It is Miocene in date. 
Whatever is the separate testimony of the vertebrates, the in¬ 
vertebrates and the plants the great interior continental planation 
on which is ushered in Tertiary sedimentation in the Black Hills 
is that recorded by the unconformity lying at the base of the 
Lance succession. Key^s. 
Basal Tertiary in Rocky Mountain Region. When Arduino 
of Padua, in the middle of the Eighteenth Century, proposed the 
geological title Tertiary, it was intended to cover the unconsoli¬ 
dated, highly fossiliferous beds which rested in marked distinction 
upon, and were in part derived from, the indurated marine strata 
which were later denominated the Cretacic. Between the terranes 
of these two periods, in England, a marked stratigraphic break 
was also early recognized, which by many English scientists was 
regarded as the greatest hiatus in all geological history. Without 
much modification of conception this latter view was transferred 
to America. But a great erosional hiatus implies of necessity 
concompetent deposition elsewhere. 
In the Cordilleran region we find something of the depositional 
equivalent of the basal unconformity of the Tertiary elsewhere. 
Along the Rocky Mountain front there occurs a full half-mile 
of strata which are older than the oldest Eocene beds. This is 
a section much more imposing than the entire Tertiary succession 
above the base of the Eocene in most other parts of the continent. 
In Colorado and New Mexico Shoshone, Denver and Ratonan 
are some" of the terms applied to different parts of this great 
pre-Eocene Tertiary sequence. There is still a major unconform¬ 
ity at the base which appears to represent even more than the 
entire Laramie sedimentation to the westward. 
