84 The American Geologist. February, i903. 
which remain nearly horizontal, except in the case of the Zia 
mesa where they dip, for the .c^reater part, southeast. The latter 
mesa is without lava deposit, except near Albuquerque and 
Ysleta ; but instead is capped with a marl which henceforth 
shall be known as the Zia marl. 1'his marl is a bluish yellow. Its 
thickness is from five to forty feet exce])t at the north to the 
west of Mesa Blanco where it thins out to an edge. 
If geologists who have visited this formation along the 
Rio Grande, were agreed we would give their conclusions and 
close the remarks on this subject: But, since one calls the 
formation Cretaceous, and another calls the lower strata Cre- 
taceous and the upper stratum "Pleistocene?,"^" and the writ- 
er's opinion dififers from either, a further discussion is neces- 
sary. No' fossils, so far as the writer knows, have been found 
in these marls. Therefore, only on stratigraphical position 
can their horizon be determined. But along this line evidence 
is not wanting. 
The Fort Union coal beds and the Puerco marls, the last 
of the Cretaceous series, underlie a thousand feet of this form- 
ation at the north along the Zia mesa break and south of Mesa 
Blanco on Avest to a point below Cabizon. It, therefore, can 
not be Cretaceous and overlie the above formations. Again, 
the marl cap is conformable with the grey and red strata be- 
neath, and these in turn lie conformable on the Puerco. In 
no place in New Mexico has Pleistocene been found lying con- 
formably on the Eocene. Not even in a single instance has it 
been found conformable on the Pliocene. The Pleistocene 
found by Cope at Placita lies unconformably on the Pliocene 
and Cretaceous. This marl cap, therefore, can not be Pleisto- 
cene. Consequently, inasmuch as the Pliocene, as we shall 
see later, lies unconformable upon it, this formation must be 
Eocene. 
The next thing to explain is why the same formation, the 
rock of the same age, to the one side of the Jemez river has a 
thick marl stratum as its top layer, and on the other side this 
marl stratum is wanting. 
As we have already noticed, practically all the strata of this 
region dip away from the Jemez mountains ; and as we proceed 
south from Jemez peak we pass over successive formations, 
all conformable, till the marl stratum at the top of the Zia 
