7IO JAMES H. GARDNER 



the Multituberculate Marsupialia, neither of which occur above, while both 

 occur below the Puerco, the Multituberculata down to the Trias inclusive. Then 

 the Placentialia are entirely peculiar in the absence of the Diplarthra and of the 

 Rodentia, orders always found in the Cenozoic beds. Then the characters of 

 the Condylarthra and Amblypoda and many of the Creodonta, which represent 

 Tertiary types, are so peculiar that we are led to suspect that when the Cretacic 

 Mammalia are fully known they cannot differ very widely from those of the 

 Puerco. 



But one area of this formation is definitely known; that is in Northwestern 

 New Mexico and Southwestern Colorado. It consists of sandstones and soapy 

 marls, and has a thickness of 850 feet. It is immediately overlaid by the Wasatch 

 Eocene, and rests on the Laramie. ^^ 



In the writings of various paleontologists since the tim^ of Cope, 

 the Puerco has been placed provisionally in the lowermost Eocene, 

 as will be referred to in further paragraphs. 



The statement has been made in geologic literature from time to 

 to time that the Puerco formation occurs in northwest New Mexico 

 and southwest Colorado; but no beds known to be Puerco in age have 

 yet been found in Colorado. This mistake arose from the work of 

 Endlich in the San Juan region in 1875.7 This was the year after 

 Cope had discovered the formation at the head of the Puerco River 

 in New Mexico to which he gave the name Puerco, and an attempt 

 was made to correlate the beds on the opposite side of the San Juan 

 Basin in New Mexico and Colorado. Holmes was working the same 

 year along the La Plata and San Juan valleys west of Endlich. Both 

 of these geologists described certain strata in their respective districts 

 as "The Puerco Marls," basing the correlations entirely on lithologic 

 similarity and stratigraphic position. It may be said in this connec- 

 tion that the term "marl," applied by Cope and others to these soft 

 argillaceous and siliceous beds, was improper, since the term, though 

 used loosely in the present day, necessarily implies strongly calcareous 

 material. 



At the time of the work of Endhch and Holmes and until the work 

 of the writer in 1907, 7° it was not known that the Nacimiento group 

 Hes unconformably on the "Laramie" and is in turn overlain uncon- 

 formably by younger formations. For this reason, it was naturally 

 assumed by the geologists of that time that beds in the same basin 

 lying next above the "Laramie" were Puerco in age. But the forma- 

 tion described by Endlich in Colorado' as Puerco is the Animas 



