308 [Dec. 7, 
Cope.] 
On the distribution of the Loup Fork formation in New Mewico. By H. D. 
Cope. 
(Read before the American Philosophical Society, December 7, 1883.) 
In his report on the Geology of New Mexico to the Secretary of the 
Interior by Dr. . V. Hayden, in 1869, this eminent geologist described 
the Santa Fé marls in their principal physical features, In 1874, in my 
report to Capt. George M. Wheeler, U.S. Engineers, I showed that this 
formation is a member ofthe Loup Fork division of the Miocene Tertiary, 
a conclusion clearly deducible from the remains of vertebrata which it 
contains. An illustrated report on the latter was published in the fourth 
volume of the report of the United States Geog raphical and Geological 
Survey, W. of the 100th meridian, Capt. G. M. Wheeler in charge (1877). 
Since that time the writer has made several visits to part of New Mexico 
not previously explored, and I am able to show that the Loup Fork for- 
mation has a much wider distribution in that Territory than has hitherto 
been supposed to be the case. 
In descending the Rio Grande, beds appear on the west side of the 
river which strongly resemble those of Santa Fé, ‘They extend along 
the eastern base of the Magdalene mountains, and as far south as Socorro, 
in considerable extent and thickness. South of Socorro they appear, but 
less extensively. The eastern part of the plain which lies. between the 
Rio Grande and the Mimbres mountains is composed of beds of this age 
where cut by the grade of the Atchison, Topeka and Santa Fé railroad, 
west of Hatch station. West of the Mimbres mountains the valley of the 
river of the same name is filled with débris of the bed of eruptive outflow 
which once covered the country, as far as t raversed by the railroad from 
Deming to Silver City. Its age I could not ascertain, 
A great display of the Loup Fork formation is seen in the drainage basins 
of the heads of the Gila river. In traveling westward from Silver City, its 
beds first appear in the valley of Mangus cre which enters the Gila 
from the east. Crossing the Gila, the mail route to the west passes 
through the valley of Duck creek, which flows sastwards into that river. 
Though bounded by eruptive hills and mountains and their outflows, the 
valley was once filled with Loup Fork beds, which have been extensively 
eroded, the principal exposures being on the north side of the valley, 
forming the foot-hills of the Mogollon range. On the divide between the 
waters of the Gila and San Francisco rivers the formation rises in bluffs 
of 300 feet elevation. The descent into the valley of the San Francisco 
brings to light a still greater depth of this deposit. The valley which ex- 
tends from the cafion which encloses the river south from the mouth of 
Dry creek to the Tulerosa mountains on the north, and between the Mogol- 
lons on the east and the San Francisco range on the west, was once filled 
with the deposit of a Loup Fork lake, This mass has been reduced by the 
erosive action of the San Francisco and its drainage, to a greater or less 
, 
