538 Arnold Heim — 



3. The fossil beds are metamorpliosed in a way which, was not 

 encountered in the true Isidro. 



4. The preservation of the fossils is different. 



5. According to Dall, the fossils would indicate the uppermost 

 Oligocene (Aquitanian) rather than the Miocene. 



It seems to be difficult to explain Nos. 1, 3, 4, by an exceptional 

 local pression of the Isidro Formation, and to deny No. 2. In every 

 case it will be very interesting to obtain the demonstration of the 

 " American " interpretation, not only by words, but with detailed 

 sections. Unfortunately, the writer's trip was too hasty to solve the 

 problem definitely. 



Monterey Formation. 



In many places along the Arroyo Cadegomo, hard, clear, siliceous 

 diatomaceous shales were found, which resemble strikingly the well- 

 known Monterey Shale (Lower Miocene) of California, U.S.A. 

 It is the writer's opinion that one day their equivalence will be 

 proven by fossils. 



Coming from the Pacific coast, this shale was first encountered near 

 the abandoned rancho San Eamon (Long. 112° 12').^ The most 

 beautiful outcrop is that of the rocks called La Ventana (the window), 

 where the shale is exposed all along the river as far up as the old deep 

 well of " Casas Viejas ", a distance of about 5 kilometres east of 

 La Ventana. 



At La Ventana (PI. XXI), the shale forms a number of graceful 

 undulations. About nine minor anticlines, with corresponding 

 synclines, were counted, with dips of the strata up to 45° or more. 

 The direction of the axes of the folds is north-north-east, and thus 

 does not correspond with the general trend of the peninsula. The 

 Ventana shales are overlain unconformably by walls of horizontally 

 bedded rough conglomerates with marine shells. The following 

 section was noted at La Ventana : — 



(1) Yellowish to white siliceous shale, schistose, containing a 

 bed of 1 foot of brown sandstone. 



(2) Well stratified siliceous shale of white, yellowish, and violet 

 colour, about 5 metres. 



(3) A massive, white, chalk-like bank of I metre. 



(4) Bed of brown flint, containing scales of ganoid fish and 

 Foraminifera (Globigerina). 



(5) Yellowish massive bed of hard, siliceous shale. 



Unconformity. 



(6) Marine conglomerate and lumachelle, with rolled fragments 

 of Lithothamniwn, probably corresponding with the Fernando 

 Formation of California (Pliocene). 



At the abandoned locality of Casas Viejas the Monterey shale, 

 in places, is intensely folded and crushed, the strike of the axes of 

 folds being north to north-east. The old well is located on the 



1 See geol. sketch map by the writer, loc. cit., 1921, pi. iv. 



