224 QUICKSILVER DEPOSITS OF TUE rACIFlC SLOPE. 



There thus seem sufficient grouiuls for asserthig that a more or less 

 continuous, but very irregular, volcanic belt stretches along the trend of tlie 

 Coast Ranges from Clear Lake' at least to the neighborhood of New Idri;i, 

 and that the eruptions, beginning in the Pliocene, extended into the recent 

 period The andesites preceded the basalts and may perhaps be consid- 

 ered as contined to the Pliocene, if indeed this period can be sharply de- 

 fined. There is considerable reason for believing that the andesitic eruptions 

 of the volcanic belt of the Coast Ranges are of pretty nearly the same age 

 as the main portion of the similar rocks of Steamboat and Mt. Shasta, and 

 that there is no great difference in age between the basalts of Steamboat and 

 those of the Coast Ranges. I by no means assert, however, that the suc- 

 cessive phases of volcanic activity were absolutely contemporaneous over 

 the whole coast. 



No uplift which the Coast Ranges have experienced compares in vio- 

 lence with that of the Post-Keooomian epoch, and consequently, whether 

 the initiation of volcanic action is referable to the very important Post- 

 Miocene upheaval or not, it is a notable fact that volcanic activity did not 

 accompany the luost profound disturbance of the Pacific Coast. The meta- 

 morphism of the rocks at the period of the Post-Neocomian upheaval, on 

 the other hand, seems reasonably ascril)al)le to the co-operation of the heat 

 thus engendered. 



During the enormous period which elapsed from the close of the Keo- 

 comian to the close of the Miocene the erosion was extremely great, yet no 

 eruptions took place. But at the close of the Miocene great masses of soft 

 sandstones were elevated, Avhich under similar meteorological conditions 

 would be eroded much more rapidly than the harder rocks of the meta- 

 morphic series. The conditions in the Coast Ranges do not, therefore, ex- 

 clude the hypothesis that the relief of pressure due to the rapid erosion of 

 these soft rocks brought about the fusion of the lavas. 



It is manifest that the eruptions took place substantiall}' along old 

 belts of uplift lines of weakness which are certainly not younger than the 

 Post-Neocomian upheaval, and, as I have pointed out on a preceding page, 



' Professor Whitne.v's p.irlics mot with no volc.aii'u- locks in the Coast Ranges inoiipr northward 

 from Clear Lake. 



