SOUTHERN CALIFOENIA. 87 



broadest dimensions, we have apparently the formation last considered represented in those 

 extensive plains which, extending inland from the sea often several leagues, give character to 

 that section of country by increasing its agricultural capacity. 



The commencement of these plains on the south may be seen, by reference to the map, to 

 correspond with the greater development of the Tertiary formation^ both in extent and thick- 

 ness. 



Thus to the N.E. of San Juan Capistrano the Tertiary deposits form an elevated mountain 

 range, attaining a height of two thousand feet or more above the sea. Here, also, are exhibited 

 the first signs of internal disturbance in abrupt and variable inclinations of the Tertiary 

 strata. 



As the necessary result of all these conditions we have a more abundant supply of material, 

 under the natural denuding influences, for the formation of the lower terraces, or the plains, 

 under consideration. 



Some of these plains, encircled by higher Tertiary hills, represent in outline beds of extensive 

 sea bays, of a previous era, now, by the elevating agencies at work, converted into their more 

 attractive land features. A fine example of this latter fact may be noticed on the accompanying 

 map as the "Santa Anna Plain." 



Desert formation. — The corresponding Tertiary formation on the eastern side of the mountains 

 must now claim our attention. And first, it will be remarked, in contrast with what we have 

 noticed on the opposite Pacific slope, that the line of junction between the crystaline rock and 

 the Tertiary belt is more distinctly marked. 



The character is well exhibited where the stream courses from the mountains enter the Desert 

 plains. They are there seen cutting their way through the Tertiary strata and presenting deep 

 vertical sections of their stratified deposits, consisting of marls, sands, and clays, with a very 

 constant accompaniment of stratified pebbles, the latter of greater or lesser thickness, and forming 

 the most usual upper capping, which constitutes the table summit of the Desert plateau. 



It is in the marl and clay deposits that gypsum makes its appearance, being frequently washed 

 out along the edges of the steep bluff's in the form of shining flakes of selenite. Here, also, 

 occur the first marine fossils found to characterize the formation, including species of ostrea. 



This marl formation, thinning out to the eastward, gives place to coarse sandy layers of great 

 thickness ; thence forming the exclusive substratum of the desert, and extending to the table 

 bluff's, which bound the alluvial bottom of the Colorado river. In this last situation it exhibits 

 a perpendicular wall 60 feet or more in height, overlaid by pebbly deposits, having an average 

 thickness of 20 feet, more or less. These pebble deposits are in this situation frequently 

 cemented by a calcareous medium more or less compact, and occasionally forming 8 close cre- 

 taceous conglomerate. 



It is in the geological features thus sketched that we can best indicate the true desert char- 

 acter of this region, covering a vast extent of country, and forming the plateau through which 

 all the rivers of this region take their course. Its deep porous layers rapidly absorbing the 

 waters of occasional heavy showers by which it is visited in the latter summer months, it 

 spreads forth at other times an arid waste often under a burning sun. The wonder is that 

 vegetation in any of its forms can pfrocure the elements for a stinted growth. 



The further relation of these strata to the supply of water for the use of travellers is of great 



