Schlosser in the Pliocene, but also all these investigators 
mistook the older for the newer formation. 
Not until 1894 was it shown by Ameghino and soon 
after by Mercerat that the Patagonian formation is in 
reality the older. In order to properly appreciate the 
existing difficulties one must remember that South 
America became isolated in the beginning of the Tertiary 
and remained so until the beginning of the Pliocene. It, 
therefore, lacks those changes in the relations to faunas 
of other territories which one finds in Europe and North 
America for the determination of the age of the dif- 
ferent formations. 
As important facts in determining the age of the 
different Patagonian strata the following may be given: 
1. The occurence of Dinosaurians and Cretaceous 
fishes in the Guaranic formation which concludes the 
Upper Chalk. The limited number of molluses at my 
disposal has not enabled me to forma decision in regard 
to the formation with Pyrotherium, which according to 
Mercerat follows, and according to Ameghino is an inter- 
calated stage of this formation. 
2. The relation of Scutella, Pectunculus cf, pulvi- 
natus, Cucullae Dalli ete. to the older European Tertiary. 
3. The occurence of maminals, which, according to 
Zittel, correspond to those ofthe older European Pliocene 
in the Araucanic layer of the Pampas formation. This 
fact, together with the occurence of typical Pampas 
mammals in North American Pliocene have largely 
established the Pliocene age of the Pampas formation. 
If, accordingly, we compare the layers containing 
Pyrotherium with the Lower, and the Patagonian form- 
ation with the Upper Eocene, and as cribe the Oligeocene 
and Lower Miocene ages to the Santa Cruz formation 
we shall probably come very near the truth. 
Of peculiar interest is the relation that exists be- 
tween the Patagonian layers of Santa Cruz and those of 
Chile, especially Navidad. According to my investigations 
