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University of California. 



[Vol. i. 



portion of the delta, it is perfectly certain that the fluviatile portion— 

 the gravel sheet of the original delta — could only have been developed 

 at base-level, that is, approximately at sea level. The level character 

 of the original gravel sheet indicates a temporarily permanent base- 

 level during the time of its formation; for its extension seaward 

 must have been gradual, the rate depending upon the climatic condi- 

 tions which controlled the supply of gravel from the innumerable 

 mountain canons which debouch upon the plain. 



This fluviatile gravel sheet of base-level origin is now 800 feet 

 (approximately) above the sea. The rear of the plateau in the vicin- 

 ity of Cajon, according to barometric observations made by the 

 writer, has an altitude of 790 feet, taking Cajon station, less than 

 half a mile distant and 463 feet above the sea, as a datum. Soledad 

 Mountain, a few miles north-northwest of San Diego, is capped with 

 these gravels, and has an elevation, according to the U. S. Coast and 

 Geodetic Survey, of 810 feet.* 



Stream Terraces. — This great delta plain has during the progress 

 of the uplift been successively terraced both by the streams and by 

 the cutting action of littoral forces. Nothing could be plainer to 

 the geological observer than these terraces and their historical sig- 

 nificance. No systematic attempt was made by the writer to deter- 

 mine their elevation. The highest stream terraces observed were 

 two on the west side of Cajon Valley, which had altitudes respec- 

 tively of 675 and 600 feet, the former a broad terrace 200 to 300 

 yards across, and the latter a narrow terrace. The stream cliffs at 

 the rear of these terraces are still quite steep to climb. From an alti- 

 tude of 600 feet down to the sea level remnants of stream terraces 

 may be found at many different levels. These terraces are usually 

 abundantly supplied with pebbles and gravel, and in this respect 

 resemble the original delta plain. The method of the develop- 

 ment of these gra v cl-strewn terraces is apparent in the canons of 

 the present base level. These streams have long since cut down to 



*It should be here mentioned, by way of qualification of this coincidence 

 in altitude of the eastern and western edges of the original gravel plain, that 

 the strata of Soledad Mountain have been locally disturbed and slightly tilted, 

 and that there are not lacking suggestions of faulting parallel with the trend of 

 the coast. That this disturbance antedated the general uplift is, however, 

 apparent from the terraces which girtle the hill. 



