CHAPTER XVIII. 



GEOLOGY OF THE DISTRICT FROM SAN DIEGO TO FORT TUMA. 



TeRTIAUIES of SAN PIECO. VoLCANlC ROCKS NEAR SAN PASftUALE. SaN YSABEL. A.XIS OP THE CORDILLERAS. WaRNEr's 



RANCHO. AgUA CALIENTE. MINERALS IN THE GRANITES OF SAN FELIPE. SPRINGS OF.. — PoRPHTRITIC ROCKS BETWEEN 



SAN FELIPE AND VALLECITAS. SpRINGS AT LATTER PLACE. ThE CORDILLERAS ThEIR BREADTH , ALTITUDE, AND STRCC- 



TURE. — Volcanic intrusions in.— Included gneiss. — Probable age of the range. — The Colorado desert. — Tertiaries 



AND ALLUVIUM. CuRVED BEDS.— EXCESSIVE TEMPERATURE. PHYSICAL APPEARANCE OF THE MARGIN. SaCKETt's WELLS, 



soil at water OF. — Igneous rocks of desert. — Lagoons. — Alamo mocho. — Wells at, and supply of water. — Slope 

 OF the desert. — Its vegetation. — Climatal phenomena. — Trail to cook's well. — Drift sands. — On new river, 

 AND the present fluviatile action of the Colorado. — Algodones. — Colorado river — Fort yuma. — Of the climate 

 and water supply of the desert. 



The low hills which surround the mis.sion of San Diego, and which form the headlands along 

 the extremities of the harhor, are of the tertiary period, and are similar in character to those 

 existing at Los Angeles and further north to the San Buenaventura river. 



These tertiaries stretch inland, forming a high terraced country, which continues to ranches 

 Solidad and Peiiascitas. Trappean and augitic dykes commence to show themselves east of 

 this, and the character of the country changes, becoming more rugged and elevated, until San 

 Pasquale is reached, where the underlying rock is a soft felspathic granite. The direction of 

 the granitic ranges here is south T0° west, cut through by a finer grained porphyry granite 

 running east and west ; here and there the granite contains hornblende crystals, but never for 

 any distance, returning again to the micaceous variety ; further east, at " Lagoons," (11 miles,) 

 it becomes coarser, with well defined crystals of orthose ; these hills enclose small valleys between 

 them, well supplied with springs, some of which, coming from deep sources, are warmer than 

 the atmosphere in winter time. 



San Tsabel mission is situated among the fertile valleys near the summit of the sierra. 

 Granite constitutes the only rock visible ; it is a coarse felspathic variety, and has masses of 

 gneiss and mica slate imbedded in it. 



From San Ysabel the trail leads through a granitic caiion, the hills on each side of which 

 cannot be less than 2,000 feet high. Near Warner's rancho is a hot spring, "agua caliente," 

 which issues out of a cleft in the granite, and has a temperature of 135° Fahr. The granite 

 between the rancho and San Felipe is soft and felspathic, with brownish mica in moderate sized 

 plates, and, as at San Ysabel, is not homogeneous, but contains masses of gneiss imbedded in 

 it ; here also occur veins of r[uartz with imperfect crystals of dark tourmaline, (schorl.) 



The valley in which Warner's rancho is situated has a slight slope to the southeast. On the 

 western side, at the base of the hills, are some marks of terrace action. 



At San Felipe, the river, which a few miles higher up was flowing, had disappeared, and its 

 course was indicated by the growth of salt grass, rushes, and a saline efflorescence of common 

 salt, gypsum, and carbonate of potass. The soil is composed of the debris of the felspathic 

 granite. The massive rock here is syenite, composed of lamellar hornblende, and white felspar, 

 with, here and there, occasional plates of mica, and some quartz. Through this a vein of 



