248 W. H. HOBBS — OklGIN OF OCEAN BASINS 



floor we have as yet no means of judging, but it is probable that the num- 

 ber stands in some such relation to those determined for the land areas as 

 do the grander seaquakes to the great land earthquakes. All the tendency 

 of recent study has been, moreover, to show that the so-called bradysisms 

 or slow movements of crustal elevation or depression take place not grad- 

 ually, but per saltum, and differ in no essential particular save that of 

 amplitude from the movements which are accompanied by earthquakes. 



The bearing of these revelations from recent earthquake study has thus 

 been to show that the formation of the present ocean basins in Tertiary 

 and later times is not only easily conceivable, but, from the data fur- 

 nished by seismology alone, extremely probable. The great Tertiary 

 mountain ranges are still pushing up their heads, and the extensive 

 studies brought together by Issel* on the basis of historical documents 

 and observed measurements show that most of the seacoasts are today 

 rising. Direct data for the continued sinking of the deeps of the ocean 

 floor are naturally difficult to secure, but, so far as measurements have 

 been made at the time of seaquakes, they indicate the main movement to 

 be a downward one. 



Crustal Block Movements within the Coral Seas 



With a view to contributing to the solution of this interesting problem, 

 the writer has brought into correlation some studies from within the belt 

 of the earth's coral seas. As all are aware, the theory of Darwin ex- 

 plains the formation of atolls and barrier reefs through the depression of 

 portions of the sea-floor, the reef of dead coral being continuously built 

 up about mountain crests within the upper 150 feet of the sea while the 

 floor is descending and submerging the mountains themselves. The later 

 theory of Murray, it has been urged with much force, owes much of the 

 support which it has secured in some quarters to the reluctance of geol- 

 ogists to accept such grand movements of the crust as are required by 

 Darwin and his follower, Dana. 



As Darwin pointed out, fringing reefs are an indication either of a 

 stationary condition or of local crustal elevation. Throughout large 

 areas of the coral seas later studies have confirmed this view, and we 

 have today the numerous accounts by scientific travelers which furnish 

 in many instances the actual present elevations of the raised coral reefs. 



With the assistance of Mr Walter F. Hunt, instructor in mineralogy 

 at the University of Michigan, the writer has brought together upon a 

 map (plate 5) the data from these sources, as well as those for sub- 

 mergence based upon the location of atolls and barrier reefs. The results 



• Loc. clt. 



