410 



PRINCIPLES OF STRATIGRAPHY 



as Tahiti could not subside even a few scores of feet without chang- 

 ing the even outHne into one of deep coves or bays, the ridges pro- 

 jecting out to sea on every side. . . . The absence of such coves, 

 on the contrary, is evidence, that any subsidence which has taken 

 place has been comparatively small in amount." (Davis-23 : j6'j.) 

 The following diagram (Fig. 83), illustrating this principle, is 

 copied from Davis. 



The Spreading Ring T*heory of Murray. Sir John Murray's 

 theory of coral reef formation is an elaboration of the early expla- 

 nation of the forms of coral reefs propounded by the poet-natu- 



FiG. 83. Block diagram illustrating the effects of subsidence of a much dis- 

 sected island around the borders of which grow coral reefs. The 

 effect of subsidence is produced by adding to the surface of each 

 successive block an amount equal to the postulated subsidence. 

 The deep embayments in the middle block, from which terrigenous 

 sediment is absent, can be produced only by subsidence. (After 

 Davis.) 



ralist Adalbert von Chamisso. It is based on the existence of sub- 

 marine banks of volcanic or other origin, rising to within a moderate 

 depth beneath the surface of the sea. The existence of such banks 

 has been abundantly shown by soundings of the Challenger and 

 other exploring expeditions. Such submerged surfaces afford a 

 habitat for lime-secreting organisms, such as deep-sea corals, echino- 

 derms, molluscs, crustaceans, etc., and their shells and skeletons will 

 gradually raise the surface of the submerged bank until it comes 

 within the zone of reef-coral growth. As pointed out by Chamisso, 

 the corals on the outer side of a growing mass are situated most 

 favorably with regard to wave activity, food supply, etc., and so the 

 reef will rise to the surface as an atoll. The nearer it approaches 

 the surface, the more unfavorable will become the atoll for the 



