TWENHOFEL: EXPEDITION TO THE BALTIC PROVINCES. 551 
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 
considerable 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 be represented by much thinner masses of stratified 
A’ 
Figure 1.— Diagram showing possible conditions in and about a coral reef. The 
line A—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 taken 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” are 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 otherwise, 
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 become almost hope- 
lessly mixed. 
