180 Davis — Dana's Confirmation of 



The diagrams by which Darwin illustrated the consequences 

 of his theory are simple transverse sections, and as such they 

 do not sufficiently represent the deductive side of the problem. 

 If they be transformed into block diagrams, in which surface 

 and section are both exhibited, their value in this respect is 

 increased ; for, as Passarge has pointed out, block diagrams 

 show what an author thinks rather than what he sees; indeed, 

 it may be claimed for them that they help their author to 

 think, even compel him to think. Therein lies half their 

 value ; while the other half lies in the ease with which the 

 reader can, with their aid, understand the thought of the 

 author. 



The accompanying block diagram exhibits three stages in 

 the subsidence of a dissected volcanic island : for convenience 

 of drawing, the island is represented as standing still while the 

 ocean rises. In the first stage, a discontinuous fringing reef 

 has been formed ; its relation to the island and to sea level is 

 shown in section on the front face of the first block. The 

 second block shows a later stage ; the size of the island has 

 now been much diminished by subsidence, and the volume of 

 the reef has been correspondingly increased in its transforma- 

 tion from fringing into barrier position ; the inferred horizon- 

 tal and slanting structure of the reef, with abundant volcanic 

 detritus in the landward part of its horizontal strata, is shown 

 in the front face of the second block : an assumed but unessen- 

 tial pause in subsidence is there indicated by a temporary turn 

 in the reef from upward to outward growth ; at the level thus 

 defined, a thin wedge of far-advancing delta deposits is inter- 

 calated in all the embayments between the earlier and later 

 calcareous strata of the lagoon, and below this level the thick- 

 ness of the outer slanting layers is increased. The proportion 

 of inner or lagoon strata, lying horizontal, to the outer talus 

 deposits, lying at a strong angle, may be varied within reason- 

 able limits. For example, in the front face of the third block, 

 where the central island has nearly disappeared, the up-grow- 

 ing reef is represented as of decreasing perimeter, with corre- 

 spondingly small external talus, thus following Darwin's 

 original diagram. But the most significant feature is yet to be 

 mentioned, although the diagram has probably already sug- 

 gested it. Darwin recognized the diminishing size and final 

 disappearance of the subsiding island as an essential conse- 

 quence of his theory : but another equally essential conse- 

 quence, of which no mention appears in his writings, is the 

 transformation of the relatively simple shore line of the initial 

 island into an indented shore line, as the sea enters the valleys 

 between the ridges of the dissected and subsiding island-mass. 

 As long as subsidence continues, no large deltas can be built 



