. W.O.Crosby — On the Appearance of a Fault. 297 



The numerous soundings made in the vicinity of reefs have shown 

 that their outer slopes are usually very steep and abrupt, often nearly 

 vertical, and in some cases probably overhanging. Although un- 

 doubtedly extremely ragged and cavernous, at all depths below one 

 hundred, or at the most two hundred, feet, these slopes are beyond 

 the influence of the waves ; and in consequence are not subject to 

 decay, except as the calcareous matter may be slowly attacked by the 

 carbon dioxide so abundant in sea- water at great depths. 



Of course when the formation of a reef first begins, it is on the 

 same level with the differently and more slowly formed but synchro- 

 nous sediments of the adjacent portions of the ocean-bed; as its 

 growth continues, however, it rises wall-like above these, and 

 synchronous layers in the two sorts of deposits, always differing in 

 thickness, are no longer stratigraphically continuous, those in the 

 reef lying at higher levels than those outside. And with the lapse 

 of time the vertical separation of beds of the same age must become 

 greater and greater ; so that if we may assume that the reef grows 

 three feet (a low estimate) while the surrounding deposit is rising 

 one foot, then when the former has attained a height of three 

 thousand feet, the latter will be only one thousand feet thick, and 

 the last-formed beds on either side will be separated vertically by 

 two thousand feet. Supposing now that the subsidence, and, as a 

 necessary consequence, the growth of the reef cease, then the gradual 

 silting up of the outside ocean will continue as before, and in the 

 course of time we will have, theoretically at least, lying on the same 

 level with the upper part of the reef formations of a much later age, 

 as shown in the diagram, where the horizontal bands within and 

 without the reef are to be synchronized according to the numbers. 



C.R.=Coral-Reef. 



The reef is elevated to form dry land, and subsequent erosion 

 developes the surface a b. Thus there is produced the general 

 appearance of a fault, but without a fracture ; and it is easy to con- 

 ceive how a geologist might be deceived and led to conclude that the 

 old coralline limestone of the reef had reached its present strati- 

 graphic position through the agency of a fault. 



In this pseudo-fault, it will be observed, the vertical displacement 

 gradually diminishes downwards, becoming zero at the bottom of the 

 reef; and many true faults, geologists are agreed, must die out below 

 the surface in a manner equally gradual. Of course observation of 



