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 7 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 ( = Xotos- 



