The Tertiary of Southern Loiver California. 541 



grained sandstone, about 20 metres. This is the base of the 

 Isidro Formation. 



4. Greenish sandstone of Isidro Formation, about 200 metres 

 or more, partly covered by Quaternary conglomerates. 



Unconformity. 



5. Comondu Formation, nearly horizontal, about 120 metres. 



6. Basalt lava flow, about 60 metres thick, the same which 

 forms the top of El Pilon. 



A similar kind of passage from the Monterey shale to the Isidro 

 Formation is seen in the river-bed above Casas Viejas, where beds 

 of white siliceous shale alternate with greenish sandstone. 



The Isidro Formation is thus apparently the normal successor 

 of the Monterey shale. 



The name Isidro is taken from the village San Isidro, which is 

 situated on the left bank of the Arroyo Cadegomo, at 3"6 kilometres 

 north-east of the old mission of La Purisima Nueva. About 3-4 kilo- 

 metres farther up the horse trail, which follows the left side of the 

 river, numerous fossils were found in an oyster-bed : Chrysodomus, 

 Psammobia, Balanus, Area, Tellina ?, Chione, Mytilus. 



Apparently the same prolific fossil beds with Pecten, Balanus, etc., 

 cross the bed of the Arroyo Cadegomo, a distance of two hours' 

 easy ride further up-stream, a little south of Huerta Vieja (which 

 means the old fruit-garden), where the Isidro Formation disappears 

 below the Comondu Formation in the direction of the interior of 

 the Peninsula. At this place, the nearly horizontal fossil beds are 

 overlain first by about 10 metres of greenish shale, followed by 

 greenish sandstone (10 metres), which in turn is covered by the 

 brownish Comondu Sandstone. 



As a whole, the Isidro Formation is formed by well stratified 

 greenish, whitish, and yellowish sandstones with interbedded 

 greenish shales. Crystals of gypsum are often found in the sand- 

 stones. The shales in places are flinty, and contain Foraminifera. 

 The total thickness of the Isidro Formation is 250 metres at least. 

 It has so far only been found for a certainty in the Cadegomo Valley 

 from Casas Viejas to Huerta Vieja. However, looking down to the 

 Gulf side from the watershed north of La Giganta Mountain, it 

 seemed that the Isidro Formation again comes to the surface along 

 the Gulf Coast north of Loreto. Possibly, the Miocene oysters 

 found at Santiago in the Cape Region, as mentioned by Gabb, 

 came from beds equivalent to the Isidro Formation. 



The lithologic facies of the Isidro Formation is neritic and 

 coastal, and thus very different from that of the Monterey shale. 

 It seems that the sedimentation occurred in warmer and shallower 

 water. The polar current may have gradually changed its course, 

 or diminished its force. 



Lithologically, the Isidro Formation much resembles the Tepetate 

 Formation, with which it was united by the author before finding the 

 fossils. Possibly, they are the same. 



