1 62 CARNEGIE INSTITUTION OF WASHINGTON. 



ocean and covers much of the San Gabriel Valley during the night. 

 But these fog-clouds rarely attain elevations exceeding 3,000 feet, 

 except when storm conditions prevail during the winter months. 

 The mountains of the Sierra Aladre range rise high above the fog, 

 and during a great proportion of the year they enjoy practically con- 

 tinuous sunshine. During the summer months the sea breeze 

 blows for a large part of the day, but it attains only a low velocity, 

 which decreases in passing from the valley to the mountain tops 

 and in going inward from the coast. 



Mount Wilson is one of many mountains that form the southern 

 boundary of the Sierra Madre range. Standing at a distance of 30 

 miles from the ocean, it rises abruptly from the valley fioor, flanked 

 only by a few spurs of lesser elevation, of w'hidh Mount Harvard is 

 the highest. Except for a narrow saddle, Mount Wilson is separated 

 from Mount Harvard 'by a deep canyon, the walls of which are very 

 precipitous. Farther to the west, beyond the saddle leading to Mount 

 Harvard, the ridge of Mount Wilson forms the upper extremity of 

 Eaton Canyon, which leads directly to the San Gabriel Valley. East 

 and north of Mount Wilson lies the deep canyon throug*h which flows 

 the west fork of the San Gabriel River, and beyond this rises a con- 

 stant succession of mountains, most of 'them higher than Mount 

 Wilson, which extend in a broken mass to the Mojave Desert. The 

 Sierra Madre range forms the northern boundary' of the San Gabriel 

 Valley, which is further protected toward the east from the desert by 

 the high peaks of the San Bernardino range. Through the Cajon 

 Pass, where the Atchison, Topeka and Santa Fe Railroad enters the 

 valley, winds from the desert frequently blow, bringing vast quan- 

 tities of dust, which sometimes diffuses through the lower air over the 

 entire valley. This dust but rarely reaches an elevation as great as 

 that of Mount Wilson, though I have seen a few wind-storms that 

 carried the dust of the desert directly over the Sierra Madre range 

 and into the valley 'below. 



For the most part, the readily accessible mountains on the south- 

 ern boundary of the Sierra Madre range have few trees near the 

 summit, and enjoy but small supplies of water. Mount Wilson 

 is remarkable in having a fine growth of trees covering its summit, 

 and in possessing within easy reach of its highest point several large 

 springs of water, vv^hich afiford a good supply even during very 

 dry seasons. 



In a dry country the question of a pure and permanent supply of 

 water is of paramount importance. It is therefore desirable to give 



