The High Altitude Conoplain — Ogilvie. 27 
THE HIGH ALTITUDE CONOPLAIN; A TOPOGRAPHIC FORM 
ILLUSTRATED IN THE ORTIZ MOUNTAINS. 
By Ida H Oijilvie, Ph. D. Rockland, Mb. 
PLATE II. 
During the past winter the writer was engaged upon a 
somewhat detailed survey of the Ortiz mountains, New 
'Mexico. These mountains are in the central part of the 
territory, some twenty-five miles east of Albuquerque, and 
somewhat farther southwest of Santa Fe. The region 
proved to be of unusual interest from the three separate 
points of view of physiography, petrography, and palaeon- 
tology. A full report on all of these subjects will be pub- 
lished elsewhere, the present paper touching only upon cer- 
tain physiographic points of general interest. 
The Cordilleras of North America in Mexico and for 
one hundred or more miles north of Mexico, consist of 
many ranges. These ranges are various in length, hight 
and direction, hut the general trend of the Cordilleras as a 
whole is N. W. — S. E. The ridges are generally steep and 
are separated by flat plateaus. The general surface of the 
plateau region is rarely less than 6,000 feet in altitude, al- 
though in some cases rivers have cut below the general 
level. 
Near the 34th parallel the Cordilleran belt divides, one 
portion trending northward, to and beyond Colorado, the 
other portion running westward and then northward 
through Arizona and Nevada. These form respectively 
the Rocky mountains and the Basin ranges. Between them 
lie the great plateaus. 
bordering the Cordilleran country are many volcanic 
areas. The eruptions vary in age and in type, extending 
from shortly after the close of the Cretaceous to nearly 
recent time, and including volcanic cones, extrusive and in- 
trusive sheets, dikes, necks and laccoliths. The volcanic 
region is confined to the borders of the Cordilleran belts. 
The Ortiz mountains lie within this borderland, in the 
eastern branch, not many miles north of the point where 
the ranges fork. They are laccolithic in origin; post-Cre- 
taceous and probably pre-Pliocene in age. West of the 
