The Tertiary of SoutJiern Lower California. 539 



undulated sliale, and is said to be drilled to a depth of 1,200 feet. 

 At the writer's visit, in 1915, 114 liters per minute of warm salt 

 water were flowing out of a 12 in. casing. According to 

 Professor Hartmann, the temperature was 34'6° Celsius, and the 

 content of NaCl 4 per mille. 



From Casas Viejas up the river, the Monterey shale remains 

 below the surface unto a place on the south side of the remarkable 

 mountain. El Pilon de la Purisima, 1 kilometre north-east of the old 

 mission, where the river makes a knee towards the north. Here the 

 shale seems to reach the surface for the last time. The overhanging 

 rock of siliceous shale of this locality will be called " Canal Head ". 

 One of the peculiarities of this outcrop is that the ordinary white 

 or grey shale contains dark layers with traces of coal. Scales of 

 fish and foraminifera are frequent again. The general dip is 20-30° 

 toward the south or south-south-west. To the north-west of Canal 

 Head the shale is somewhat faulted and crushed. 



The base of the shale of Canal Head is formed by a basic intrusion, 

 which has been described and figured in an earlier paper,^ while at 

 the top a gradual passage to the Isidro Formation has been noted 

 (Fig. 3). 



The thickness of the Monterey shale, on account of its irregular 

 folding, could not be measured, but it certainly exceeds 50 metres, 

 and may measure several hundred metres. The lithological facies 

 corresponds exactly to that of the Californian Monterey shale, which, 

 in the writer's opinion, is a deposit of cold ocean water. Indeed, the 

 work of the Challenger expedition by the late Sir John Miirray 

 has plainly shown that the diatomaceous oozes of the present time 

 are confined to the circumpolar regions. It seems thus, that in 

 Miocene time a current of cold water was running down the Pacific 

 coast as far as Southern Lower California. The regularity of 

 stratification and the scarcity or absence of coarse psammitic 

 intercalations seem to indicate a rather deep ocean bottom during 

 the sedimentation of the Monterey shale. In every case, the 

 Californian facies of this shale is in a great contrast with the facies 

 of the younger Miocene formations (Isidro, Comondii), to which 

 nothing similar is known in Upper California. 



Isidro Formation (Miocene). - 



The normal base of this formation was observed at Canal Head 

 of La Purisima (Fig. 3), where the following succession was noted, 

 beginning with the base : — - 



1. Intrusive basaltic rock. 



2. Monterey shale as described above, about 50 metres, passing 

 gradually into 



3. Greenish shale and shaly sandstone, with layers of fine- 



^ See photograph, pi. ii, fig. 2, in the writer's publication on the volcanoes, 

 loo. cit., 1921. 



2 " Griine Molasse " of the writer's paper, loo. cit., Ifll6. 



