84 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 13 



submerged mountains, i.e., Angel de la Guardia, San Lorenzo and 

 Ceralbo. Others, as Tortuga may not have appeared until the marked 

 diastrophism of the Upper Pliocene and Pleistocene, when the peninsula 

 gained its present elevation and general outline. San Jose, Carmen, San 

 Marcos, and other islands close along the peninsular shore, may be fault 

 splinters, and their developments intimately related to the dynamics of 

 this compensatory zone. Too little is known geologically to time events 

 in detail. 



It was not until the Pleistocene that the modern peninsula arrived. 

 During the late Miocene and the early Pliocene most of the modern 

 southern half of the peninsula was covered by sea, judging from the 

 sedimentary beds. The marine formation described by Darton as "the 

 yellow beds" (Jour. Geol. 24:720-748. 1921), is particularly significant. 

 The beds appear to have covered all but the several old central igneous 

 masses along the Sierra Giganta axis south of San Ignacio and a couple 

 of western outposts. Hence it would appear that for a portion of the 

 Upper Tertiary, the peninsula south of Sierra Calmalli was represented 

 only by a series of islands, where Tertiary pediments crested the invad- 

 ing sea, and which are still represented by locally exposed schists and 

 granites about the bases of modern mountains (Darton I.e. fig. 3). 



The more important of such postinsular and prepeninsular masses 

 appear to be represented by the Sierra Vizcaino, the Magdalena Island 

 area. Sierra Zacatecas, a segment of the Sierra Giganta about Cerro 

 Giganta east of Comondu, and the Cape District. A shallow portal 

 across the mid-peninsula about the latitude of San Ignacio appears to 

 be clearly defined and to have been contemporaneous with the yellow 

 beds. As the Giganta fault subsequently became active, the modern pen- 

 insula grew southward by anticlinal uplift and accompanying pyroclas- 

 tics, which now form the greater part of the higher land. Today we see 

 that the whole eastern side of the peninsula from San Ignacio south 

 was tilted upward, the great Giganta scarp reared, the Pliocene waters 

 retreated from the southern and western borders and from the trans- 

 peninsular portals. The mountains of the Cape District and the Sierra 

 Vizcaino complex were tied onto the peninsula. This was not accom- 

 plished as one gradual sea recession, but according to local orogeny and 

 to the eustatic periods of the Pleistocene. Hence such lowlands as the 

 Vizcaino Depression were alternately opened and closed. The expanse 

 of sand in remotely serried dunes appears to have been formed by suc- 

 cessive beaches. In summary, there is little or nothing in the stratigraphy 

 to indicate that the southern half of the peninsula was anything more 



