a, 
—_ eee 
TWENHOFEL: EXPEDITION TO THE BALTIC PROVINCES. 349 
and even fathoms,” ! and these depressions of the surface afford 
lodging places to the multitude and variety of organisms, which live 
in, on, and about a growing coral reef, a variety of life to which atten- 
tion has been called by nearly every student of coral reefs. 
Many feet, in the vertical sense, may and usually do separate the 
upper surface or summit of a coral reef from the bottom of the sea 
about the margin, while the horizontal distance between the summit 
of the reef and the bottom is usually not great. Conditions of this 
nature make it possible for individuals of the same species to live on 
top of the reef among the coral colonies and also twenty-five to a 
hundred feet lower down only a few feet distant horizontally, and it is 
certain that some of the shells of animals living on the top will be 
washed into lower waters by the waves which dash over the reef during 
high tide or during storms. Since variation of depth up to one 
hundred feet is not sufficient to control the distribution of a great 
number of species of marine organisms, it follows that many species 
will thrive in multitudes on the top of the reef and over the adjacent 
bottom. 
Ultimately the reef may and probably will become surrounded with 
sediments of an age somewhat younger than that of the reef on the 
same level with itself. Stating the matter differently, the rocks com- 
posing the reefs would have their time equivalents in strata holding a 
lower vertical position in the section. 
Exactly similar conditions must have obtained among and about 
many, if not all, of the ancient coral reefs and the writer is quite 
positive that they existed in connection with the reefs of Gotland, 
which stood above the bottom as is proven by the fact that cases are 
visible where coral colonies have fallen from the sides of the reefs to a 
lower level. That the tops of these elevations were irregular and filled 
with deep and shallow holes is shown by the included masses of clay, 
many of which are filled with excellently preserved fossils. 
If the organisms of this sea were distributed over this sea bottom 
both upon the tops of the reefs and the lower levels about their margins 
there should be some evidence of this distribution in the existing fossils. 
In this consideration the distribution of the corals themselves has no 
validity, because in the cases of most of them they range more or less 
throughout the entire section. The chief reliance has been placed on 
the brachiopods. The natural stratigraphic position of Bvlobites 
bilobus (Linné) is in the lower half of the Gotland section, but near 
1Dana. Loc. cit., p. 145. 
