Tlie Tertiary of SontJiem Lower California. 535 



Hilario, the underlying strata of the Tepetatc Formation come 

 to the surface, at the centre of a very gentle and wide dome-like 

 anticline. These strata show the lithological characteristics of 

 the typical Monterey Shale of California : a white siliceous shale, 

 porous at the surface, containing brown fish scales. 



It is generally known that the Monterey shale of the California 

 districts belongs to the Middle or Lower Miocene. But this Shale 

 of San Hilario is separated from the overlying Tepetatc Formation 

 by neither a thrust place nor an unconformity. The succession 

 of strata west of San Hilario was noted as follows, from below : — 



1. Siliceous shale, about 20 metres ; base, not visible ; grading 

 into 



2. Brown, hard, stratified sandstone with moulds of marine 

 bivalves [Chione) ; thickness 2 to 3 metres. 



3. Grieenish, more or less sandy shale ; thickness 50 to 100 

 metres. 



4. This shale No. 3 seems to be normally overlain by the 

 Orthophragniina sandstone as described above. 



The question which arises from the above data so far remains 

 unsettled : Is the siliceous shale of San Hilario to be regarded as 

 an Eocene precursor of the Monterey shale of California, or did 

 Ortho])hragmina Pratti survive for one or two geological periods 

 longer in this country than elsewhere ? 



The Tepetate Formation is widely distributed all around El 

 Tepetate, forming the low mountains up to the top. Again, it 

 was observed 16 kilometres north-west of El Tepetate at the 

 Arroyo del Liebre. 



Farther north-west along the big bend of the Arroyo de la Salada 

 in the region of the Rancho El Sauce (Lat. 24° 35', Long. 111° 28'), 

 underlying the Salada Formation with a perfectly visible 

 unconformity, sediments were found which are identical litho- 

 logically, for the most part, with those south-west of El Tepetate. 

 It is true that I did not find Orthophragmina here, but this may be 

 due to insufficient collecting. Gray sandstone with tracks of 

 animals, ripple-marks, and concretions is exposed at the Eancho 

 Santa Rosa. Not far from this place, at Santa Teresa, a soft, 

 somewhat leafy, greenish sandstone with small fragments of coal 

 was found. A small hill to the south-west of that jolace is formed 

 of violet, pure clay containing crystals of gypsum and shells of 

 small Foraminifera, while still farther south-west follow rusty 

 sandstones with ochre layers. It seems that all these strata belong 

 to the Tepetate Formation. 



All around the above-named places along the big river bend at 

 El Sauce, the dip seems to be more or less uniform, averaging 

 20° to 22° toward a little east of north (Fig. 2). With a surface 

 of three kilometres in the direction of the dip, the thickness of the 

 Tepetate Formation thus must be at least 1,000 metres. 



The facies of the Tepetate Formation seems to be chiefly neritic. 



