238 WHITMAN CROSS 
geologic history, though imperfectly. The divisions were not 
intended to be ‘faunal divisions’’ as Mr. Willis states, but 
geological divisions. They were founded primarily on the evi- 
dence of fossils, because lithologic character utterly failed to 
indicate the importance of the events which must have occurred 
before and after the Rico epoch. The lower line of the Rico 
formation is accurately determinable in the field so far examined 
by the relations of athin black limestone, a thin-bedded series 
ot sandstones, fifteen feet in thickness, above it, and a reddish 
sandy-looking limestone. The thin black limestone is a guide 
because it is full of a foraminifer, Pusudina cylindrica, common 
in the Hermosa limestones and absent from the Rico beds; the 
other limestone because it is crowded with the Permo-Carbon- 
iferous fauna. The upper line of the Rico formation, as at 
present mapped, expresses the fact of the apparent limitation of 
the Rico fossils. The succeeding strata have been referred pro- 
visionally to the Triassic system because no known evidence 
gives a basis for recognizing the Permian or any other geologic 
unit expressive of the important events which clearly must have 
occurred between the Rico epoch and the deposition of the fos- 
siliferous Triassic strata. The fact that a lithologically hetero- 
geneous section must be scrutinized most carefully to detect 
planes of relative geological importance is well illustrated in this 
series of ‘‘Red Beds” above the Rico formation. The Rico 
fauna is a marine fauna; the vertebrates and invertebrates of 
the Triassic beds, one thousand feet above, are fresh water or 
land forms. No more marine sediments occur in the section of 
this region until the Upper Cretaceous rocks are reached. It is 
not now known whether strata of Permian age are present in 
this section or not. If that question is ever settled it will be on 
fossil evidence. Ultimately, the knowledge will be acquired 
indicating further subdivision of this section in order to express 
the sequence of events so imperfectly recorded in the rapidly 
and irregularly changing lithologic character of the strata. 
The absolute necessity of using fossils in discriminating geo- 
logic formations is frequently illustrated in the areal mapping 
