u8 



University of California. 



[Vol. x. 



Southern California Railway which runs from Oceanside to Escon- 

 dido. Towards the ocean the lower terraces usually break off 

 abruptly in a vertical sea-cliff above the present strand. Towards 

 the mountains the upper plateau passes in between and behind the 

 outlying isolated peaks and ridges of the range. The contrast in 

 the topography at the line where the plain encircles the mountains 

 is very glaring and impressive. The mountains are bare, jagged 

 .masses of white-weathering granite and gneiss, utterly devoid of 

 soil, and sustaining only a very scant shrubbery. The mesa plain 

 presents what is locally a level surface, with no' appreciable irregu- 

 larities to break its surface. Standing on the surface of this plain 

 at its highest part, say near Cajon, or on some of the granite ridges 

 which project through it, the mesa may be seen to extend north 

 and south along the coast as far as the eye can reach. The moun- 

 tain slopes pass down beneath it and present in a remarkable way 

 the effect of a partially buried topography. As soon as one rises 

 above the level of the plain and looks down upon it, it is discovered 

 that the mesa is dissected by numerous canons and valleys, through 

 which run the streams from the mountains to the sea. The chief 

 of these streams are the San Diego, the San Diegito, and the San 

 Luis Rey. The streams are the chief agencies which have effected 

 the terracing of the mesa, although true sea-terraces may also in 

 several places be readily discriminated. The mesa with its terraces 

 is being cultivated agriculturally, and as occupation of the land 

 proceeds, and water is brought to it, it will undoubtedly become a 

 rich country. 



Structure. — The mesa is genetically a composite Pliocene delta. 

 Structurally it consists of two easily observable portions, viz.: (i) A 

 great thickness of marine sands, generally very light colored, and 

 more or less coherent, passing into sandstone. (2) Above these 

 sands and sandstones a comparatively thin sheet of river gravels of 

 dark appearance, generally somewhat rusty and more or less firmly 

 cemented, so as to constitute a true conglomerate. Mixed with the 

 gravel is much sand and silt. The disintegration of this conglom- 

 erate gives rise to the soil of the country, which is well adapted for 

 fruit culture. Over considerable areas the surface is more silty and 

 sandy than gravelly; but the upper sheet of gravel is a character 



