TERTIARY AND LATER FORMATIONS 609 



region the ''Laramie" is succeeded unconformably by the Animas forma- 

 tion, which has been regarded generally as the time equivalent of; the 

 Denver. Some geologists still refer these formations to the Cretaceous, 

 but G. B. Eichardson (122) has recently shown in a manner rather con- 

 vincing to many geologists that the Denver formation is of Tertiary age. 

 He referred the Dawson arkose to the Eocene, and since, according to 

 Doctor Knowlton, the "leaves from the lower part of the Dawson . . . 

 are undoubtedly Denver in age," the Denver must of necessity be of the 

 same age as the Dawson, namely, Eocene. Eocks that seem to be equiva- 

 lent in age to the Animas rest unconformably on the "Laramie" near 

 Dulce, New Mexico, and farther to the south in the San Juan Basin the 

 oldest Tertiary rocks — the Puerco formation — lie unconformably on the 

 "Laramie" in some places and on the Lewis shale in other places (117). 



In the Eio Puerco field rocks that are lithologically similar to the 

 Wasatch of the San Juan Basin rest unconformably on the Mesaverde. 

 They are varicolored, with yellow, pink, and red shales predominating, 

 and although they have never been measured they are comparable in 

 thickness to the Tertiary formations of that basin. These rocks consist 

 of poorly consolidated sandstone, conglomerate, and shale. The upper 

 part is light colored and seems to be referable to the Sana Fe marls, the 

 type locality of which is a few miles east of the Eio Puerco field. 



No rocks referable to the Tertiary were found in the Tijeras field, but 

 in the Hagan field a series of beds comparable in ^ thickness, lithologic 

 character, and general appearance to the Tertiary of the Eio Puerco 

 field rest unconformably on the Mesaverde. These beds contain great 

 quantities of petrified wood, but no fossils were found in them that make 

 possible the determination of their age. The Hagan and Eio Puerco 

 fields are only about 30 miles apart, and it is difficult to believe that the 

 rocks of these two fields, which are comparable in every other way, differ 

 in age to any great extent. 



In the Cerrillos field rocks that are lithologically similar to the Ter- 

 tiary rocks of the Hagan and Eio Puerco fields and of the San Juan 

 Basin lie unconformably on the Mesaverde. This is known as the Galis- 

 teo sandstone. The lower part of this formation consists of conglomer- 

 atic sandstone and shale identical in character and appearance to those of 

 the Hagan field 10 miles to the west, but the upper parts are coarser and 

 contain large angular blocks of many kinds of rock, including quartzite, 

 and crystalline rocks such as are now exposed in the mountains a few 

 miles to the northeast ; limestone, containing Carboniferous fossils, sand- 

 stone, etcetera. Eock formations, from which these were probably de- 

 rived, are now found upturned and eroded on the flanks of the moun- 

 tains. The Galisteo sandstone is manv hundreds of feet in thickness and 



