226 
IOWA ACADEMY OF SCIENCE 
of Rio Arriba county, and the extreme eastern part of San Juan 
county. The present areal extent of the marls is only a small por- 
tion of the formation as originally deposited. The existing rem- 
nant has been preserved only on account of its position in the 
bottom of a syncline, the general erosion of the region not yet 
having reached a level that is beneath the bottom of the terrane in 
the central part of the trough. No other areas or exact equivalents 
of the Chaco formation are known elsewhere within the boundaries 
of New Mexico. 
The fact that there are marked unconformable relationships 
between the beds of the Nacimientan and Chaman series already 
indicates that a considerable section of Eocene sedimentation is 
missing in the region. This may mean that all of the Chicka- 
sawan stage of the standard American section is represented by 
an erosion interval and that the beds in question belong to a later 
stage of the Eocene than has been supposed, corresponding perhaps 
to the Claibornian stage. Of the European analogues the latest 
Suessonian or earliest Parisian may be considered the nearest 
equivalents. 
No strata of Oligocene age have been as yet identified within 
the boundaries of New Mexico. The Eocene and Miocene deposits 
of the region appear to be separated by an important erosion in- 
terval. 
The Arriban series of marls is one of great thickness. Its best 
known member is the Santa Fe marl, although the formation never 
has been carefully delimited. 
The title of Santa Fe marls was first used as a geological term 
by Hayden* in 1869. He applied the name to certain marls, clays 
and sands which he found in the Rio Grande valley north and south 
of the city of Sante Fe. Cope f subsequently studied the vertebrate 
fossils of these marls and referred the formation to the Loup Fork 
Tertiary. A few years later Stevenson 1 noted these beds, but, as 
recently shown by Johnson, 8 his name Galesteo group covers the 
same section as Hayden’s title. In a later publication of the 
geology of the Cerrillos hills, Johnson 11 has extended Hayden’s title 
so as to include also the Quaternary wash-deposits. The upper 
limit of the Santa Fe marls should probably be regarded as prop- 
erly located at the base of the vast basaltic flows of the region. 
* U. S. Geol. Sur. Terr., Prelim. Field Kept., p. 66, 1869. 
fProc. Acad. Nat. Sci. Phila. , Vol. 26, pp. 147-152, 1874. 
JU. S. Geol. Sur., W. 100 Merid., Vol. Ill, Supp., pp. 159-163, 1881. 
g Sell. Mines Quart. , Vol. XXIV, p. 316, 1903. 
I! Loc. cit. 
