546 Arnold Heira — 



The Pliocene of Poza Griande is covered by loose sands with, marine 

 shells of recent type, the " Medanos " of the Mexicans. 



From this place farther north the Salada Formation is found in 

 every river bed near the Pacific coast : at Arroyo Comondu (25° 49'), 

 Arroyo Pabellon (25° 57'), at the Eanch San Eamon of Arroyo 

 Cadegomo (26° 5'). At this latter place the green sandstone of the 

 Salada Formation is capped by a white wall of shell breccia (Fig. 7). 

 This calcareous horizon was followed by Mr. E. Kluth some 6 kilo- 

 metres farther north-west to the mouth of the Arroyo San Gregorio, 

 where numerous fossils were collected {Balanus, Polinices, 

 Chione, etc.). 



The question of termination inland of the Salada Formation 

 along the Arroyo Cadegomo has not been solved definitely. 

 Possibly the marine conglomerate and agglomerate of shells at 

 La Ventana (PL XXI, Fig. 2) represents the shore deposit of the 

 Salada Formation. In the higher Mesa conglomerates, more inland, 

 no more marine shells were found. They seem to be of terrestrial 

 origin, and possibly belong to the beginning of the Quaternary 

 epoch. It is possible also that they are the terrestrial equivalent of 

 the Salada Formation. 



Pleistocene. 



The Pliocene strata are covered along the Pacific coast by sand- 

 hills called Medanos. They form soft waves, which may rise up 

 to about 100 metres above sea-level. Marine shells of recent type 

 are found frequently, like Area tuhereulata Sby., Donax cf . eayennensis 

 Lam., Tivela bryoniana {radiata Dall), oysters, etc. (det. B. L. Clark). 

 It seems that the Medanos, now quiet and covered with cactus, 

 have been wandering dunes during Quaternary time. 



For the terrestrial Quaternary see the author's publication of 

 1916. 



Tectonic Movements. 



We have mentioned the intense folding of the Purisima nueva 

 Formation and the Monterey Formation in the region of La Purisima. 

 This substructure of the Mesas is in disharmony with the general form 

 of the Peninsula (PI. XXII). The axes of the folds are directed 

 towards the north or north-east — generally north-north-east, i.e. 

 oblique to the direction of the Peninsula. 



The Isidro Formation (Miocene), in places being conformable with 

 the Monterey, the folding of the Monterey must coincide with the 

 vast unconformity below the Comondu Formation. The age of the 

 folding thus is probably the later 'part of Miocene. 



After the deposition of the Comondu Formation, very gentle 

 folding has continued locally, as, for instance, west of La Purisima 

 (PI. XXII). The youngest folded formation found is the con- 

 glomerate (Pliocene ?) of Casas Viejas on Arroyo Cadegomo, where 

 a little anticline was observed with dips up to 5°. This folding is 

 a continuation of a fold of the underlying Monterey shale, the 

 anticlinal dip of which reaches 15°. 



