Recent Birthing of Ocean-level. 257 



shift. Until the delicate problem of the relation between the emerged 

 forms and the corresponding sea surface has been worked out, both 

 as to general criteria and as to the local application of those principles, 

 the actual amount of delevelling registered in the Pacific Islands 

 cannot be definitely stated. 



California. — Jajaan, Alaska, and California are examples of the 

 lands where local movement of the earth's crust, upwards or 

 downwards, have taken place during the last few decides. Strand- 

 marks of an ocean-level, which some thousands of years ago was higher 

 than now, must there have altitudes different from contemporaneous 

 markings in more stable regions. Identification of the 20 ft. 

 strand is thereby made specially difficult. Yet Lawson's observations 

 on the emerged zone at Carmelo Bay and Monterey, California, 

 are suggestive for future research. He notes that the emerged 

 terrace reaching 25 or 30 feet above sea-level is the " most defined 

 and persistent elevated terrace in this coastal region ", and concludes 

 that the sea beat on the higher shore about as long as it has worked 

 at the existing level. ^ Has the emerged terrace a height slightly 

 greater than that of the Micmac and other terraces because California 

 has been specially affected by diastrophism during the recent period ? 



Amount of the Eustatic Change. 



Without further multiplying examples, those so far mentioned 

 may be reviewed conveniently by the use of the following table.^ 

 Its full meaning cannot be understood without recalling certain 

 principles. In each case where the author of a record uses " sea- 

 level " or " water level " as his only datum, reference to the 

 more significant and less equivocal high-tide level is not directly 

 possible. The fifth column of the table therefore has many blanks. 

 A wave-cut bench is likely to give too low a value for the emergence ; 

 a storm-beach, too high a value. Coral reefs give too low a value 

 unless they can be shown to have grown up to the sea surface (mean 

 or high-tide level) before the emergence began. Even in that 

 instance their crests may have been somewhat abraded by storm 

 waves just after the sea-level began to sink, for a dead coral reef 

 is a friable structure. Special note should be taken of W. B. Wright's 

 view that the British 25 ft. beach has been warped .° 



In spite of all uncertainties, columns 4 and 5 of the table show 

 that the negative eustatic shift of ocean level which suffices to explain 

 the facts, is close to 20 feet. It may have been a little less ; it was 

 probably not as much as 25 feet. 



^ A. C. Lawson, Bull. Dep. Geol. Univ. California, vol. i, 1893, p. 53. 



- In a verbal communication Dr. E. 0. Hovey has stated that at many 

 parts of the Greenland coast he observed beaches with crests 15 to 20 feet above 

 mean sea-level. 



^ See reference above, and also Geol. Mag., Vol. viii, 1911, p. 99. 



VOL. LVII.— NO. VI. 17 



