146 WALLACE W. ATWOOD 



tain. Others have become isolated and stand 5-15 feet above their 

 surroundings. Some appear to have recently lost their capstones 

 and stand as unprotected spires. Others have long since lost 

 the protection offered by the blocks that caused their develop- 

 ment, and are now reduced to mounds. In portions of this area these 

 mounds resemble an irregular grouping of old-fashioned bee-hives. 

 The work of erosion goes on very slowly now, but may be assumed 

 to have been more rapid during the moist climate of Pleistocene 

 •times. 



A summary. — Based on the above data the history of Red Moun- 

 tain may be sketched as follows: Late in the Tertiary period a 

 secondary volcanic vent opened on the outskirts of the San Francisco 

 Mountain center. A series of explosions occurred, building up a 

 cone of fragmental material to a height of several hundred feet. The 

 first material ejected fell on a relatively flat surface. As the cone grew 

 in size, the fragmental material rested at higher and higher angles. 

 Lavas rose into the crater, and volcanic bombs were formed. The 

 winds carried much of the loose material to the northeast, building 

 that portion of the crater rim highest. Late in the growth of the 

 mountain a small amount of lava issued from the crater and flowed 

 part way down to the southwest slope. Eruptions continued until the 

 mountain rose about 1,000 feet above the plateau level. Based on 

 the number of layers in the tuff, a conservative estimate of the num- 

 ber of explosions necessary to have built the cone is between 4,000 and 

 5,000. This estimate does not allow for any height above the present 

 summit, and is therefore probably far below the correct figure. 



Waters that were presumably associated with each eruption and 

 more recent rains assisted in cementing the fragmental material 

 together. Rain and running water have now partially dissected the 

 cone, exposing the successive layers of fragmental products and 

 developing a large variety of pecuhar forms. The remarkable exposure 

 on the northeast side extends nearlv to the core of the mountain. 



