756 GEORGE DAVIS LOUDERBACK 



graphic peculiarities along the rift line of the recent great California 

 earthquake indicate that faulting has taken place in recent time 

 along the same line, and probably several times. In the great faults 

 along the fronts of faulted ranges (the so-called Basin range type) 

 the evidence is to the effect that the movements were repetitive and 

 of moderate extent. Why should these phenomena take place in small 

 spurts, with perhaps several years or even centuries of quiescence 

 between ? Why should not the mountain blocks rise, or the valley 

 blocks fall at once with a great crash ? 



For a long period (post-Phocene, however) a large part of the 

 California coast (several hundred miles in extent) has been rising with 

 respect to the sea. Its upward progress is marked by raised beaches 

 and sharply incised terraces traceable to about 1,500 feet above the 

 present sea-level. At San Pedro Hill Professor Lawson identified 

 eleven terraces between sea-level and 1,240 feet. On San Clemente 

 Island he determined eighteen between sea-level and 1,500 feet. ^ The 

 upward movement has evidently not been uniform, but periodic, 

 periods of activity being followed by periods of quiescence. And this 

 is apparently true of all similar crustal disturbances. 



Furthermore, the progress of older earth movements as preserved 

 in the records of sedimentation, deformation of strata, and uncon- 

 formities, shows that discontinuous movements and periodicity have 

 characterized diastrophic history from the earhest geological times. 



We should naturally expect, then, that vulcanism, if it is simply 

 one phase or accompaniment or result of diastrophism, would partake 

 of that universal and perhaps most striking characteristic of diastrophic 

 processes, the alternation of periods of visible activity with periods of 

 apparent rest, and the accomplishment of any general change by suc- 

 cessive small increments, rather than by one great catastrophic effort. 



What success shall we have if we try to get a concrete conception 

 of a series of eruptions from a reservoir caused by locally concen- 

 trated radioactivity ? A cubic mile of radium, or perhaps of pure 

 pitchblende, if properly blanketed with rock, would probably in the 

 course of time melt itself and its immediate surroundings, but we 

 should have an eruption largely of radium or of pitchblende. How 



^Lawson, " Post-PKocene Diastrophism of the Coast of Southern CaHfornia," 

 Bulletin of the Department of Geology, University of California, Vol. I, pp. 115-60. 



