twenhofel: expedition to the Baltic provinces 



;J51 



attempted upward growth a part of the materials composing the sedi- 

 ments. It became possible, then, for very small thicknesses or even 

 small patches of material in the upper portion of a reef to represent 

 consideraljle thicknesses of the sediments which were accumulating on 

 the flanks of the reef. Stating the matter differently, in the early 

 stages of reef development, great thicknesses of unstratified reef 

 limestone should lie represented by iinuli thinner masses of stratified 



Figure 1. — Diagram showing possible- conditions in and about a coral reef. The 

 line .\-A' represents sea-level during the development of the reef and the deposits 

 about it. The line B-B' represents the present surface. Sediments other than 

 those of the reef are stratified. The unmarked portions of the reef above the 

 present surface are assumed to have been eroded away after uplift had taiken place. 

 Unmarked portions between the present surface and the former sea-bottom, M and 

 M", are assumed to be filled up with sediments which were developed after the 

 reef was formed and hence are younger than the reef. 



M-M" are deposits of the same age. On the margins of the reefs the stratifica- 

 tion is inclined. 



C-C" Eire colonies of the same species of organism on the reef and adjacent bot- 

 tom. Sediments containing the shells of these species will be of the same age, 

 but the reef rock on each side and above the colonies of the reef is older than the 

 sediments which fill the cavities. 



b" represent the base or beginning of a coral colony. 



a-a' are stratified sediments within the reef. The time equivalents of these 

 sediments are below M and M". 



P-P are outcrops of inclined stratified rock with everything else, other than 

 some exposures of the reef rock, hidden. The exposures might readily lead to the 

 conclusion that the reef on the left is older than that on the right, and that the strata 

 are progressively younger toward the right or, if the fossils indicated otherui^^e. 

 that a fault lay between the two reefs. 



limestone lying at a lower level, and, after the reef reaches the water- 

 level, almost negligible thicknesses of reef limestone should find their 

 equivalence in much thicker beds of stratified limestone, also at a 

 lower level. Further, in all the stages of reef development the later 

 animals would live above the shells of the earlier animals, and also in 

 the hollows of the reef below them, and when the whole became turned 

 into stone the shells of different times would liecome almost hope- 

 lesslv mixed. 



