The Tertiary of Southern Lower California. 545 



Miocene age. Along the Pacific coast, however, marine formations 

 of Pliocene and Pleistocene age have been found in a great extension. 

 The most perfect section encountered is that of the cattle ranch 

 called La Salada, at the left bank of the Arroyo de la Salada (Fig. 5). 



The Salada Formation of La Salada River forms a low and unfolded 

 plateau, which extends from La Salada Ranch as far as 15 to 20 kilo- 

 metres inland, while, on the other hand, it was followed along the 

 coast as far as Arroyo Colorado, lat. 24° 15', a distance of 

 45 kilometres, wherefrom it extends still farther south-east for an 

 unknown distance. 



The lithological facies shows typical shallow-water conditions of 

 sedimentation, as indicated by numerous fossils of coastal type, lilce 

 Balmius, and layers of conglomerate. The fossils are abundant. 

 However, shark teeth and Balanus excepted, they are badly pre- 

 served. Casts were collected of large Polinices, Conus, Chrysodomus, 

 Calliostoma, Oliva, Turritella, Chione, Tellina, Mytilus, etc. The 

 stratigraphic position, and the fossil evidence seems to indicate 

 that the Salada Formation is of Pliocene age. 



The rim of the Mesa de la Salada is formed by an unfossiliferous 

 limestone (No. 10 of Fig. 5), which seems not to be of organic origin. 



S. N. 



Mesa 50-80 m. 



Fig. 6. — The younger Tertiary at Rancbo San Ramon, near mouth of Arroyo 

 Cadegomo. 1-2. Salada Formation (Pliocene). 1. Greenish and yellowish 

 soft sandstone. 2. White limy agglomerate of shells. 3. Mesa con- 

 glomerate (Pleistocene). 4. Low terrace conglomerate, Younger 

 Quaternary. 5. Alluvial soil of river bed. 



Along the Arroyo de la Salada the Mesa rises gradually toward 

 the interior, beginning with a rate of 6 to 7 metres to 1 kilometre. 

 The thickness of Salada Formation being some 30 metres only, the 

 Salada Formation terminates toward the interior by erosion, its 

 substratum (Tepetate Formation) coming to the surface (Fig. 2). 



Farther north, along the Pacific Coast, a very small outcrop at 

 the mouth of the Arroyo Soledad (lat. 25° 11') excepted, the Salada 

 Formation is hidden below the large areas of marine Pleistocene and 

 alluvial plains, as far as a place called Poza Grande at the Arroyo 

 San Benancio (lat. 25° 45'), where the upper calcareous part of the 

 Salada Formation reaches the surface again. The beds here are 

 formed of sandstone and agglomerate of broken shells. The upper 

 strata contain many marine gastropods, the shells of which are 

 covered with peculiar Bryozoa. Pholad borings are found at 

 the base. 



VOL. LIX. — NO. XII. 3.5 



