The Tertiary of Southern Lower California. 547 



Also faulting has occurred since the deposition of the Comondu 

 Formation. Some 6 kilometres upstream from San Isidro, before 

 coming to Paso Ancho, a fault cuts ofi the Isidro Formation against 

 the Comondu, while the overlying basalt, of probably older 

 Quaternary age, is undisturbed (Fig. 8). 



Simultaneously with the Miocene folding a general uplift 

 commenced. Indeed, the peneplain which corresponds with the 

 unconformity below the Comondu Formation must have been 

 preceded by such a general rising of the land. Since younger Miocene 

 times the interior of Southern Lower California seems not to have 

 been submerged again. 



S. N. 



El Devisadero. 

 Trail. 

 rr . Arroyo Cadegomo. --^,.QL 



Base line about 100 metres above sea-level. 



Fig. 7. — Section of Miocene at San Isidro, about 6 kilometres N.E. of Mission 

 La Purisima. I. Isidro Formation, with Oyster and Turitella banks + 

 C. Comondu Formation. 



The uplift was gradually increasing towards the east, as shown 

 by the following observations : — 



1. On the watershed (La Giganta), the conglomerates of younger 

 Miocene and Pliocene are still pointing gently upwards in the 

 direction of the Gulf. The pebbles came from the Gulf side. This 

 shows that the Gulf was caused by a post-Miocene subsidence 

 (see writer's publication of 1916). 



2. Marine Pliocene and Pleistocene formations were deposited 

 along the Pacific coast. 



This uplift which has caused the immense table lands still continues 

 in the present time. Marine shells are found on the Medanos at least 

 up to 100 metres above sea-level. The river banks are cut into the 

 alluvial coastal plain (region of Magdalena Bay). The great " big 

 meadow " or Llano de Yrais, on the east side of Magdalena Bay, 

 formerly a lagoon of 100 square kilometres, is dried up, and the 

 muddy ground of Magdalena Bay in places already reaches the 

 surface at low tide. 



The close relation to the folding shows plainly that the general 

 uplift and the faulting are not independent phenomena, but 

 consequent to the same tangential force which had caused the folding. 

 The post-Tertiary uplift of the peninsula seems to balance the 

 subsidence of the Gulf. 



