228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



Another kind of evidence is the presence undersea of steeply slop- 

 ing areas of bottom between flat plain surfaces, thereby indicating 

 the submergence of an escarpment or steep slope formed by marginal 

 cutting of the sea, at a time while the land stood high enough for the 

 shore to have been at the escarpment front. (See p. 236 of this 

 article for additional consideration of this criterion.) 



The presence below sea level of deposits of peat composed of the 

 remains of land plants is still another kind of evidence. Submerged 

 peat deposits of this kind occur in St. John Harbor, Antigua, and 

 near Key West, Florida. 



There are other kinds of evidence, but the four mentioned are suffi- 

 cient for present purposes. Nearly all of the important living off- 

 shore reefs of the world have now been investigated with reference 

 to evidence of submergence, and it has been found that practically 

 all, if not actually all, have formed after an episode of submergence. 



-2L. 



200l 



600'. 



Old oonte 



^S!^d#L_. ^^^^ ,..----— --■-■--^^E""- S_^alevej 



f^-'-:^>'' ^WSir^VV -v>^ L ^?^^?^^^Submerged Terrace , ? ft. s 

 Solution wells 6-7/ 2 faihoms ^^ — -^ *— ,CTms ' 



deep through oolite \ 



Solution well 31-33 fathoms deep \ 



.100 fms. 



Horizontal scale 

 ;. ooo o zfloo Agog 6,000 Feet 



eotfJ 

 Fig. 11. — Diagrammatic section across the barrier reef, Andros Island, Bahamas. 



In the study of fossil reefs the principal criterion for inferring 

 whether they were formed during the submergence of their basements 

 is the nature of the contact between the fossil reef and the immedi- 

 ately underlying geologic formation. If the underlying formation 

 has an uneven upper surface and if pebbles derived from it are incor- 

 porated in the overlying reef, the deduction is considered warranted 

 that the basement stood above sea level long enough for it to have been 

 eroded by atmospheric agencies and that it was then brought below 

 sea level before the reef occurring above it began to form. Contacts 

 of the kind indicated are called unconformities. For a number of 

 years I have been studying not only the corals in the fossil reefs of 

 the West Indies and the southern United States, as may be seen by 

 referring back to pages 221, 222 of this paper, but also the geologic 

 relations of the reefs, including the nature of their basal contacts, 

 and the results are given in the following tables : 



