226 The American Geologist. October, 1894 
Reptiles and particularly the dinosaur group ore eorrelatid in 
development with the growth of the peneplain. 
Reptiles proper are the best indices we have of the influ- 
ence of the peneplain on organic life. "This class," says Le 
( !onte,* "seems to have culminated about the end of the Juras- 
sic or the beginning of the Cretaceous period. If their re- 
mains are more abundant in the Jurassic in Europe, they are 
far more abundant in the Cretaceous in America. In 
fact,*' continues this lucid writer, "we had here in America 
during that time an extraordinary abundance and variety of 
reptilian life, including all the principal orders." With the 
abundance of land reptiles we find also the greatest variety of 
marine forms. "The reptilian type," states the venerable 
Dana,f "was unfolded in its complete diversity: the sea, air 
and earth had each its species; and there were both grazing 
and carnivorous kinds, of large and small dimensions - ." The 
chief of these in our interest are the Dinosaurs. "This great 
group of reptiles," says Marsh, J "were the dominant land an- 
imals of the earth during all Mesozoic time. According to 
present evidence, the dinosaurs were confined entirely to the 
Mesozoic. They were abundant in the Triassic, culminated 
in the Jurassic, and continued in diminishing numbers to the 
end of the Cretaceous period, when they became extinct."' 
Reptilia are characteristic lowland forms. They will endure 
the cold of high altitudes and latitudes only by falling into a 
state of torpidity. In the development of the peneplain from 
the high relief of the Permian, and again, at the close of the 
Jura-Trias, the widening out of the lowland, with plains and 
jungles near tide-level, followed by depression of the land, 
must have highly favored the water-loving reptilia. It is to 
these geographic circumstances. I think, that we must look 
for an explanation of the remarkable history of this class in 
.Mesozoic times. 
Objections arising from imperfect Tcnowledge of the extent of 
the peneplain. The most serious objection which, as it appears 
to me, can be raised against the view that the production of 
broad lowlands in the middle Mesozoic favored the domi- 
*Elements of Geology, third ed., 1891, p. 484. 
{•Manual of Geology, third ed., 1880, p. 485. 
JAm. Jour. Sri.. HI. vol. xxxvn. 1889, p. 'VM. 
