CHAPTER XI. 
FOSSILS OF THE MESOZOIC PERIODS, WITH A SKETCH OF 
THE GEOLOGY OF THE MESOZOIC AND TERTIARY 
BEDS OF NORTHERN NEW MEXICO. 
The remains of vertebrate animals described in the following pages 
were derived from five horizons, two of which are Mesozoic and three Ter¬ 
tiary. Of these, the former are the supposed Trias, and the Cretaceous of 
the Niobrara group, or No. 3 ; the latter are the Lower or Wahsatch Eocene, 
the Loup Fork beds of the Rio Grande Valley, and the Placita marls of 
“Postpliocene” age. 
As preliminary to the description of the fossils, a general account of 
the positions and relations of these beds is given* in the order of age. The 
fossils of the Trias are few, and were obtained on the western side of the 
Gallinas Mountains. Those from Cretaceous beds were mostly obtained 
from outcrops along the eastern base of the Rocky Mountains in Colorado. 
1.- THE MESOZOIC BEDS OF THE WESTERN SLOPE OF THE SIERRA MADRE. 
The close of Chapter H of my preliminary report to you (see Annual 
Report Chief of Engineers, 1875, p. 988) describes the first appearance of the 
variegated red and yellow beds of Mesozoic age, as the exploration was carried 
from the valley of the Rio Grande to the dividing axis of the Sierra Madre. 
As these strata- rise, forming large hills on the north side of the Rio Chama, 
* See Annual Eeport of the Chief of Engineers, 1875, II, p. 981, for nay account 
of the geology of Northwestern New Mexico. 
