496 A. Tylor — For?nation of Deltas. 



possible tliat pieces of rock, with, living coral attached to sea-weed or 

 wood, might be thus floated from one island to another, etc. Also the 

 fundamental rocks, on which, the coral reefs are attached, may have 

 been of such a friable character as not to be able to stand above the 

 level of the sea. Some such conditions may have prevailed in the 

 Carboniferous epoch. We have no modern instance of such rocks, 

 neither have we evidence in former periods of any coral deposit of 

 the form and dimensions of the Pacific Islands. Modem peat is not 

 a type of Coal. Geologists assume too much when they rely on the 

 analogy of modern conditions being a complete key to the knowledge 

 of former conditions on the earth. Nor is the reverse true. 



There are fourteen cases of elevation recorded by Mr. Darwin 

 varying from 20 to 30 feet above the sea-level to a maximum of 

 300 feet. Very few of these instances have been examined by any 

 geologist. There is, therefore, no means of measuring the thickness 

 of the upheaved coral deposits by sections beyond 300 feet, as there 

 are no known cases of elevation beyond that limit of thickness. 



Mr. Darwin writes, p. 146, Geological Observations, 1851. In 

 the 4th chapter the growing powers of the reef-constructing polyps 

 were discussed, and it was shown that they cannot flourish beyond 

 a very limited depth. "In accordance with this limit there is no 

 difficulty respecting the foundations on which fringing reefs are 

 based ; whereas with barrier reefs and atolls there is a great apparent 

 difficulty on this head : in barrier reefs, from the improbability of 

 the rock of the coast, or of banks of sediment, extending in every 

 instance so far seaward within the required depth; and in atolls from 

 the immensity of the spaces over which they were interspersed, and 

 the apparent necessity for believing that they are all supported on 

 mountain summits, which, although rising very near to the surface 

 level of the sea, in no one instance emerge above it. To escape this 

 most improbable admission, which implies the existence of submarine 

 chains of mountains of almost the same height, extending over areas 

 of many thousand square miles, there is but one alternative, namely, 

 the prolonged subsidence of the foundations on which the atolls were 

 primarily based together, with the upward growth of the reef-con- 

 structing corals. On this view every difficulty vanishes. Fringing 

 reefs are thus converted into barrier reefs, and barrier reefs, when 

 encircling islands, are thus converted into atolls the instant the last 

 pinnacle of land sinks beneath the surface of the ocean." 



The maximum depth of the coral lagoons is 60 fathoms, either in 

 lagoons or between the shore and barrier reefs. In dredging outside 

 coral reefs you have a sandy bottom, and not a coral bottom, at 

 greater depths than 60 fathoms, and not often deeper than 20 or 30 

 fathoms. Then comes a very steep descent to very great depths, 

 almost close to the coral islands. 



Mr. Darwin and Sir H. T. De la Beche differ on the important 

 point of there being no existing case of a sea-bottom of large extent 

 sufficiently shallow to form a base for coralline zoophytes to work 

 on. The following extract is most clear on this point : — 



De la Beche writes, Geological Observations, p. 227, 1851 — " As 



