Waring — Lavas of Morro Hill and Vicinity. 103 



tance to the northeast — at the mouth of l^ig^er Canyon, on 

 Temecnla River, — there is a small area of lava agglomerate 

 which may prove on detailed examination to be similar in 

 character to the tuff southeast of Morro Hill. 



1^0 positive criteria for judging of the period of intrusion 

 were found but the rounded shape of Morro Hill is indicative 

 of submerged erosion. The elevations of the bottom of the cliff 

 of Morro Hill and of the top of the hill to the southeast are about 

 the same. It therefore seems possible that the steep, cavernous 

 western slope may indicate wave-action and that the top of 

 Morro Hill stood above a Tertiary sea-level at a time when the 

 lower hill was planed down to that sea-level. As the sedi- 

 mentary deposits along the nearby coast are believed to be 

 of Eocene age it seems probable that the intrusion was pre-Ter- 

 tiary. 



Along the crest of Santa Kosa Mountains, about 15 miles 

 north of Morro Hill, there are four or more mesas formed by a 

 sheet of basaltic lava. A few miles farther north, on the east 

 slope of the mountain, there are two other small areas of sim- 

 ilar rock. The occurrence has been described by Fairbanks* 

 who says that the sheet which has been eroded into the several 

 detached mesas is about 100 feet thick at its eastern side and is 

 underlain by 200 or 300 feet of sandstone that rests on the 

 granitic and metamorphic basement rocks. To the west the 

 lava has been worn thin by erosion. A notable feature of this 

 lava region is a narrow flow that extends for a vertical distance 

 of 1800 feet down the southern slope of Avenaloca Mesa or 

 Cienaga Peak. The lava of these mesas is in large part 

 coarsely vesicular, but contains dense, massive portions. The 

 two small areas on the mountain-side north of the mesas are of 

 basaltic material that is finegrained and in places conglomer- 

 atic. A slide made of rock from the northernmost basaltic 

 occurrence, l-J miles south of Wildomar, shows it to be an 

 olivine basalt. It has a groundmass of fine crystals of labra- 

 dorite interspersed with magnetite. Olivine crystals, somewhat 

 altered, are irregularly distributed throughout the rock. 



Between the two northern basaltic areas Fairbanks mentions 

 a ridge of tuff intersected by dikes, and says that this ridge 

 seems to be of an older period of effusion. Fairbanks (pp. 78- 

 80) has also described areas of basaltic lava in and near Jacumba 

 Valley in southeastern San Diego County, and areas of vol- 

 canic tuff farther east and northeast on the slopes leading down 

 to the Salton Basin. He considers both these lavas and those 

 in the Santa Rosa Mountains to be of late Tertiary age. 



* Fairbanks, Harold : Geology of San Diego County, also portions of 

 Orange and San Bernardino Counties. Eleventh report Cal. State Mineral- 

 ogist, pp. 101-104, 1893. 



Am. Jour. Sci.— Fourth Series, Vol. XLIV, No. 260.— August, 1917. 



