838 A. W. GRABAU — PALEOZOIC CORAL REEFS 



1901.^ Since then I have received from Dr Carl Wiman a copy of his 

 paper on Siluric coral reefs in Gotland, published in 1897, in which he 

 describes reefs of a similar character. These will be referred to again. 

 Coral reefs of similar types have been described also by Dupont, from 

 Devonic and Carbonic limestones of Belgium. During the summer of 

 1902 I had an opportunity, in connection with my study of the Michigan 

 limestones, to visit the Niagara coral reefs in the vicinity of Milwaukee, 

 and later I found similar structures in the Onondaga limestones of Wil- 

 liamsville. New York. The examples studied may now be described in 

 detail. 



Description of Coral Reefs 



hamiltoy reefs of alpexa, michigan 



The reefs in the vicinity of Alpena are best exposed in the quarries 

 opened in the Alpena limestone, which has a thickness of about 35 feet 

 and is the middle member of the Hamilton or Traverse group in the 

 Thunder Bay region. Reefs occur in higher and, to some extent, in 

 lower strata of the group, but none of these are well exposed. 



In outline the reef is roughly dome shaped with slopes sometimes as 

 great as 30 or 40 degrees. The height of the dome is equal to the thick- 

 ness of the limestone stratum, about 35 feet in this region, and the great- 

 est diameter, which is near the base, is perhaps several hundred feet. 

 Thechief reef builders represented areFavosites, Acervularia,and Stroma- 

 topora, which form the main mass of the reef, while between them are 

 found the smaller corals and brj^ozoa, as well as brachiopods, crinoids? 

 and a few other types of organisms. There is an absence of stratification 

 in the central reef mass, the structure being exceedingly irregular. Be- 

 tween the corals and shells is found a filling of coral sand, which gen- 

 erally consists of rather coarse fragments with a predominance of crinoid 

 joints. Solution and recrA^stallization have not infrequently taken place- 

 with the result that dogtooth spar is of common occurrence. 



The coral heads are generally of large size; sometimes they are over- 

 turned, but most of them appear to lie in their normal position of 

 growth. In some places the crystalline coral sand forms most of the 

 reef exposed, the large coral heads being scattered through the sand. 

 The sand shows no stratification so far as observed. The sand filling the 

 cavities of the reef is generall}^ much coarser than that forming the 

 normal sediments on its flanks. In places at some distance from the 

 center of the reef the rock consists of a breccia made up of brachiopods, 

 bryozoa, and the small branching corals, with a plentiful interspersing 

 of the joints of crinoid stems. 



* June, 1902, p. 176. 



