The character of the material making up the Deseado deposit, its variations in size and material, the presence of worn pebbles and bits of bone, show these layers to be a 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 deposits 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 intermittent stream, such as occur in arid and semiarid countries. 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 bedding 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 following table. ----------+----------------------------+-----------------+---------- | AMEGHINO, | WILCKENS, | ORTMANN, | 1906[3] | 1906[4] | 1901[5] ----------+-----------------+----------+-----------------+---------- Lower | terrestrial | marine | | Miocene +-----------------+----------+ | | | |Patagonian |Patagonian | | |(transgression) | ----------+-----------------+----------+-----------------+---------- Oligocene | | |Deseado | | | |(regression) | ----------+-----------------+----------+-----------------+---------- Eocene |Santa Cruz | | | |Notohippus | | | |Astrapothericulus|Patagonian|Casamayor | |Colpodon | |(regression) | ----------+-----------------+----------+-----------------+---------- Upper |Deseado |Sehuen | | Cretaceous|Astraponotus | |St. George | |Casamayor |Salamanca |(transgression) | | |Roca | | | |Luisa | | ----------+-----------------+----------+-----------------+----------