Recent Sinking of Ocean-level. 251 



has been made for the difference in estimating former sea-level 

 from emerged beaches. 



Variation in the position of wave-base is worthy of special 

 attention. Waves running in on a flat shallow shelf are damped, 

 so that wave-base approaches a minimum depth inshore. Wave- 

 base is deeper along a steejD-to shore, other things being equal. In 

 the former case the foot of the sea-cliff is characteristically at or 

 near high-tide level ; in the latter case it is typically well below 

 high-tide level, as exemplified in Samoa, where many headland cliffs 

 plunge into water about 2 fathoms deep. Generally old strand-levels 

 on oceanic islands are likely to be notably higher than the 

 corresponding wave-cut benches, as well as 2 to 10 or more feet 

 lower than the level crests of storm beaches. 



4. Wherever a coastal region is now undergoing, or has recently 

 undergone, a crustal movement, the search for local evidences of 

 an earlier eustatic sinking of ocean level may give negative or 

 doubtful results. The marks of the old strand may be drowned, 

 or else elevated to heights which are not to be directly correlated 

 with the heights of the old strand in undisturbed rocks. 



5. In special cases the eustatic shift may locally cause a change 

 in the tidal range, affecting the datum if this be high tide. 



6. Finally the height of the old strand line above mean sea-level 

 may depend on the cause of the eustatic shift. Suppose, for 

 example, that a shift of level averaging 20 feet were -due to a recent 

 increase of the ice-cap on the Antarctic continent. Near the ice-cap 

 sea-level would be kept, through gravitative attraction by the new 

 ice, a little higher than it would be if abstraction of water from the 

 ocean were the only cause for the change of level. In the northern 

 hemisphere the lowering of sea-level would be all the greater because 

 of attraction to the new ice. 



For many reasons, therefore, the search for complete proof of 

 the eustatic shift here postulated is bound to be prolonged. Mean- 

 time a preliminary study of some records made in separate mutually 

 distant regions has been made, with results that also show the 

 hypothesis to merit discussion and further testing. Certain 

 examples may now be briefly reviewed. 



British Isles. — The raised beaches of Scotland and Ireland are 

 €elebrated, and like the American cases already noted, occur in 

 an area of uplift following deglaciation. In Scotland the uplift was 

 differential, but according to most published statements the lowest 

 beach is at nearly constant level, which is sometimes described as 

 about 20 feet or about 25 feet above Ordnance datum. Ordnance 

 datum is mean water-level at Liverpool, so that the beach varies 

 from 15 to 20 feet above local high tide, according to these state- 

 ments. Geikie writes : " The most prominent of all the raised 

 beaches is that which lies at a height of about 20 or 25 feet above 

 high water . . . varying in breadth from 6 or 7 miles to not, more than 

 a few feet." Along its inner edge are sea-cliffs, caves, sea-stacks, 



