498 EMMONS AND MERRILL — SKETCH OF LOWER CALIFORNIA. 



passed over more hurriedly, the following brief outlines of the geological 

 structure have been gleaned. 



The most prominent geological feature is formed by what he calls the 

 mesa sandstones, which lie for the most part in a comparatively undis- 

 turbed position and rise gently eastward toward the summit of a mesa- 

 topped range running parallel with the Gulf shore, then descending in 

 an abrupt deeply eroded escarpment for 10 or 15 miles to the coast. The 

 long interior valleys, the most southern of which he describes as extend- 

 ing from La Purissima to Todos Santos, a length of 150 to 200 miles, 

 with an average width of 10 miles, are apparentl}^ eroded out of the mesa 

 sandstones. These sandstones are coarse grained and highly metamor- 

 phosed along their eastern edge when traversed by eruptive rocks. They 

 also frequently contain eruptive material, pebbles and bowlders of erup- 

 tive rock sometimes forming the bulk of their material. Over these, 

 sometimes showing direct unconformability, lie fine-grained argillaceous 

 sandstones and shales, with a calcareous and highly fossiliferous stratum 

 near the top containing casts of living species of shells which he con- 

 siders of post-Pliocene age. In the mesa sandstones no fossils were found 

 by him, but from a few fossil oysters sent in from cape Saint Lucas he 

 assumes a probable Miocene age. The post-Pliocene sandstones usually 

 lie on the lower margins of the mesa on either coast, covering the eroded 

 mesa sandstone to an elevation of several hundred feet above the sea- 

 level, and are more continuous on the west than on the east coast, ex- 

 tending, according to him, with few interruptions, the entire length of 

 the coast from Magdalena bay to cape Colnett. The mesa structure is 

 interrupted at various points by cross-ranges, of which the most notable 

 is the recent volcanic belt extending westward from Los Tres Virgines in 

 latitude 27° 30', and a mass of older granite mountains near the east 

 coast which in places project above them about the latitude of 28° and 

 apparently extend up to the southern border of the region examined by 

 the writers. 



Gabb also mentions recent gravel formations filling the lower valleys, 

 which are perhaps more recent than the post-Pliocene beds and which 

 also form well defined terraces northward from Todos Santos (of the 

 south) 60 feet high. 



Of the southern point of the peninsula he says : 



''All the higher ridges are made up of granites and syenites, and formed, during 

 the deposition of the heavy bedded mesa sandstones, an island of considerable 

 height and very irregular outline." 



Associated with the granites are mica slates which apparently form the 

 country rock of the metalliferous veins of this part of the peninsula. 



