twexhofel: 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. 



INIany 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 Bilobites 

 hilohus (Linne) is in the lower half of the Gotland section, but near 



1 Dana. Loc. cit., p. 145. 



