646 INDEX TO THE STRATIGKAPHY OF NOETH AMERICA. 



form of long fissures, forming dikes, which, having spread their surplus over the surrounding 

 plains, have closed, never again to reopen. In this manner immense areas have been covered 

 with caps of eruptive rocks often 100 feet thick, the source of which is now entirely hidden, an 

 occasional hint only existing in the denuded section of some bluflf where the dike has been cut 

 through by the agency of running water. 



The post-Pliocene rocks usually lie on the lower margins of the mesa in such a manner as 

 to show that they were deposited during the period of elevation of this portion of the peninsula. 

 The older Mesa sandstones are usually so little disturbed that the two formations seem conform- 

 able, though sufficient evidence exists to prove that the elevating force had been acting for a 

 long time before the oldest beds of the newer formation were deposited. This later series con- 

 sists of fine-grained argillaceous sandstones and shales, some coarser light-gray sandstone, and 

 lastly a thin bed, highly fossiliferous, as are also some of the earlier strata, but the latter highly 

 calcareous. Where the series remaias unbroken, this last stratum is always the highest, and it 

 is nearly made up of the casts of living species of shells, Ostrea cummingii being almost the only 

 one retaining its structure. At Purissima, on the west slope, the Mesa sandstones have been 

 folded in a series of long and graceful undulations, the tops denuded to a nearly straight line and 

 the post-Pliocene lies unconformably capping the surface. On the opposite side of the moun- 

 tains bordering the guK there are still more marked instances of unconformability, which will be 

 described in their proper place. 



The Mesa sandstones are easily distinguished from the overlying rocks by their coarser 

 grain, greater compactness, and above all by their being highly metamorphosed along the greater 

 part of their eastern margin. Another marked feature is the presence of large quantities of 

 bowlders and pebbles of volcanic rocks embedded in them, sometimes to such an extent as to 

 form even a preponderance of the bulk of some strata. These bowlders are unifiormly small and 

 very much rounded near the west coast wherever the rock is encountered, and increase in size 

 toward the vicinity of Loreto, or rather toward that part of the coast a little below Loreto, in 

 such a manner as to point unmistakably to this region for their origin. Not only does the size 

 increase, but in the same ratio is the increase in number and the decrease in the amount of 

 attrition to which they have been subjected. The lithological characters vary markedly from 

 those of any eruptive rocks encountered in place on the peninsula; no similar rocks have been 

 discovered between the Mesa sandstones and the underlying granite, and the only reasonabl-e 

 conclusion which can be arrived at is that they must have been derived from a body of land 

 wliich formerly lay in that region now occupied by the gulf, and somewhere in the vicinity of or 

 a little south of Carmin Island. 



Another striking feature of this region is the peculiar manner of the elevation of the mesa. 

 It has not been lifted by an evenly distributed force; not, like most mountain chains, by a folding 

 along a given axis. The eastern side seems to have felt this force almost alone, the elevation 

 of that portion lying to the west seeming to be due almost as much to the rigidity of the ro'cks 

 as to the extension westward of the uptilting power. More properly speaking, the great force was 

 exerted very nearly parallel with what is now the coast line of the guK, and from there toward 

 the Pacific this agency diminished so gradually as to produce no breaks or even foldings worthy 

 of mention. We thus have the whole width of this portion of the peninsula tilted up by its 

 edge from coast to coast, so that traveling eastward one can hardly realize the rise until, within 

 15 or 20 miles of the coast, he finds himself on the verge of a precipitous descent of from 2,000 to 

 3,000 feet in height. This eastern escarpment extends from near La Paz to near Moleje, with 

 but few interruptions, and exhibits nearly everywhere the projecting edges of nearly horizontal 

 beds of sandstone, sometimes unaltered, but usually metamorphosed. 



The preceding description of the peninsula by Gabb should be supplemented 

 by consulting the articles by Lindgren, and by Emmons and Merrill. Lindgren's 

 account is quoted below and the map follows his inference in placing the "Mesa 

 sandstones" in the Cretaceous. 



