A RIVER DEPOSIT 9 
water deposit. The absence of any marine fossil in a bed 
otherwise rich in fossils indicated that it was a fresh water 
formation. ‘The crossbedding, the irregularity of the de- 
posits and the mud balls, prove that it was the work of a 
river. As there are no aquatic forms in the fauna I further 
conclude that it was the deposit of a temporary or inter- 
mittent stream, such as occur in arid and semiarid coun- 
tries. ‘The layer could hardly be interpreted as a part of 
a flood plain; for it is very limited in extent, there being 
bluffs on three sides of our exposure, but in them no trace 
of the Deseado was found, nor was I able to pick up the 
formation again across the Chico River. Then the bed- 
ding is very irregular, much more so than is typical of flood 
plain deposits. The conclusion I reach then is that this 
Deseado pocket represents the bottom of an ancient 
stream, which flowed over a land surface made up of the 
white sandy clays of the St. George age. 
The age then of the Deseado beds must be older than 
the Patagonian, and younger than the white sandy clays 
of the St. George. 
As to the age of the Patagonian two very divergent 
positions have been taken, which may be best indicated 
by the diagram on page 10. 
Without going into the history of the various positions 
which different authors have taken, and which will be 
found given in detail in Wilckens’ paper, or in less detail 
in Ortmann’s, we will consider the positions of the most 
recent students of the question. Ameghino postulates a 
marine and a continental series of deposits being laid down 
more or less simultaneously. In the marine series below 
the Deseado, which is grouped as Guarantic, he places 
the Luisa, the Roca and the Salamanca, followed by a 
hiatus, then the Sehuen, which in turn is followed’ by an- 
other hiatus and the end of the Cretaceous is reached. 
The Patagonian is his Eocene. Parallel to the marine 
series is the terrestrial, where the Casamayor (= Notos- 
