Cope.] "Uo [Dec. 7, 



On the distribution of the Loup Fork formation in New Mexico. By E. B. 



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. F. V. Hayden, in 1869, this eminent geologist described 

 the Santa Fe 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 of the 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 Geographical and Geological 

 Survey. W. of the 100th meridian, Capt. G. M. Wheeler in eharge(1877). 



Since that time the writer has made several visits to parts 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 Fe. They extend along 

 the eastern base of the Magdalena 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 Fe railroad, 

 west of Hatch station. West of the Mimbres mountains the valley of the 

 river of the same name is filled with debris of the bed of eruptive outflow 

 which once covered the country, as far as traversed 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 creek, 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 eastwards 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 canyon which encloses the river south from the mouth of 

 Dry creek to the Tulerosa mountains on thenorth, 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 



