FOSSIL REEFS: STLURTC 



421 



structural character and composition can I)c readily ascertained. 

 The materials of which they consist are i)artly corals, among which 

 are Halysites and Favosites, both massive and branching forms, 

 Striatopora, Pachypora and Syringopora, which latter is especially 

 adapted by its loose method of growth to retain the coral sand pro- 

 duced by the erosion of the reef. Stromatopora heads are like- 

 wise common in these reefs, and often of large size, and with them 

 occur calcareous algse ( Sph?erocodium ) and Spongiostroma, which 

 sometimes constitute as much as 50 per cent, of the reef-building 

 types, though when occurring alone they form beds instead of 

 reefs. 



On the borders of the Gotland reefs characteristic interfingering 

 of the organic and detrital limestone is often well shown, as for 

 instance at the cliiT of Hoburgen in the southern part of the island. 

 Often masses of stratified sand appear to be included in the reef 

 mass, a phenomenon also observed in the reefs of Wisconsin. The 



Fig. 85. Diagram of the reef structure at Gotland, and the relation of the 

 reefs to the bedded formations. (After Wiman.) 



detrital lime sand is thickest on the border of the reef, and it is 

 often well stratified, dipping away from the reef as in the Wiscon- 

 sin examples. Overturned coral heads are common on the border of 

 the reef, these being torn from their original anchorage and rolled 

 about by the waves of the Siluric sea, which used them as mill- 

 stones to grind the coral and other organic rock into sand and 

 mud, which was either deposited within the interstices of the reef 

 or swept outward to form bedded strata of limestone. The detrital 

 lime sand is in part the result of such destruction of the corals and 

 the brachiopod and molluscan shells, but to a large extent it is 

 derived from the dismemberment of crinoids, the stems and calices 

 of which yield an abundance of discs and plates through the natu- 

 ral dissociation of these components on the maceration of the 

 dead organism. Much of this crinoidal detritus is already of suffi- 

 ciently small grain to be directly included among the stratified 

 sediments forming outside the reef. Crinoidal conglomerates, 

 formed of the discs of the larger crinoid stems, as well as coral 

 conglomerates, are of common occurrence on the borders and sum- 

 mits of the reefs. 



