282 Dr. J. D. Dana on the Origin of 



vations have great interest, but they only prove that, in coral- 

 reef seas, corals will grow over any basis of rock that may 

 offer where the water is right in depth, and do not nullify any 

 of the evidences of subsidence. This point should be kept 

 before the mind in all future study of coral-reef regions. 

 Borings in coral-islands, as recommended on a former page, 

 are the true means of investigating it. 



b. The old hypothesis that atolls may have been built upon 

 the summits of submerged mountain-peaks, or volcanic cones 

 at the right distance under water for growing reef-corals, or, 

 if not at the right level, brought up to it by other organic 

 depositions, or down to it by abrasion, is urged by Mr. 

 Murray. 



This writer observes that " the soundings of the c Tuscarora ' 

 and i Challenger ' have made known numerous submarine 

 elevations ; mountains rising from the general level of the 

 ocean's bed at a depth of 2500 or 3000 fathoms, up to within 

 a few hundred fathoms of the surface." But " a few hundred 

 fathoms/' if we make few equal 2, means 1200 feet or more, 

 which leaves a long interval yet unfilled *. 



It is also urged that some of the u emerged volcanic moun- 

 tains situated in the ocean-basins " may have been wholly 

 swept away and left with a few fathoms of water above them. 

 But this is claiming more from the agents of erosion than 

 they could possibly have accomplished, as the existence of an 

 atoll in the ocean and the examples on coasts of wave and 

 tidal action prove. 



D. To give completeness to the hypothesis which makes 

 barrier and atoll islands out of submarine banks (whether 

 these banks have a basis of volcanic or other rocks, or of 

 calcareous accumulations), it is necessary to show that the 

 waters of the waves and currents can make barrier islands and 

 atolls out of such banks without subsidence ; and explanations 

 to this effect have been given. 



It is urged, in agreement with Darwin, that the outer por- 



* The actual depths over the elevations in the ' Tuscarora' section between 

 the Hawaian Islands and Japan, numbering them from east to west, are 

 as follows:— 1,11,500 feet; 2,7500 feet; 3,8400 feet; 4, 12,000 feet; 

 5, 9000 feet (this seven miles west of Marcus Island) ; 6, 9600 feet. 

 Whether ridges or peaks the facts do not decide ; probably the former. 

 No. 1 has a base of 185 miles, with the mean eastward slope 40 feet per 

 mile (= 1 : 132), and the westward 128 feet per mile. No. 2 has a breadth 

 of 396 miles, with the mean eastern slope mostly 37 feet per mile, but 51 

 feet toward the top, and the westward, 55 feet per mile ( = 1 : 96). No. 3 

 was the narrowest and steepest, it being about 100 miles broad at base, 

 and having the mean eastern slope 192 feet per mile and the menu western 

 200 feet 



