FOSSIL REEFS: DEVONIC 



427 



characteristic of the structure of PalcTOzoic reefs is shown. The hest 

 examples are found in the eastern area of the northern part of the 

 Southern Peninsula in the vicinity of y\lpena. Here their occur- 

 rence is marked in the topography by low mounds or swellings which 

 rise above the general level of the country, a feature observed in 

 Siluric coral reefs as well, as, for example, in the neighborhood of 

 Visby, on the Island of Gotland, where such knolls form a marked 

 feature of the topography, many of them being surmounted by 

 windmills. 



The Alpena reefs (Grabau-40) are mostly well exposed in the 

 quarries which are extensively opened up in this rock, the purity of 

 which makes it desirable for chemical and other purposes. Analyses 

 of the reef rock have given over 99% of calcium carbonate. The 



Fig. 88. Diagram of the reef structure in the Traverse (^Middle Devonic) 

 limestones of Alpena, Mich. The black masses represent corals 

 (Favosites, Acervularia, etc.) and stromatoporoids. The dip of 

 the calcarenytes and calcilutytes on the margin of the reefs and 

 the interfingering of the reef and clastic formations are shown. 



reef mounds are of the usual lens shape, the height averaging 35 

 feet, while their diameter at the base is several hundred feet. The 

 chief reef-builders are large heads of Favosites, Acervularia, and 

 stromatoporoids, many of them three or four feet in diameter, 

 while occasional stromatoporoids up to 10 feet in diameter also 

 occur. Branching species of Favosites are common, and bryozoans, 

 crinoids and brachiopods help to swell the organic content of the 

 reef. Between these organisms the coral sand abounds, binding the 

 whole into a solid, structureless mass, in which lines of stratification 

 are entirely wanting. Even where the coral sand fills extensive 

 cavities in the reef mass, no stratification is shown. In such cases 

 the coral heads may be found wholly enclosed in the sand, and 

 often overturned. For the most part, however, the coral heads lie 

 in their normal position of growth, successive heads growing on a 

 foundation of older corals embedded in crystalline sand. (Fig. 88.) 

 On the borders of the reef-mounds the clastic lime sand forms 

 beds which are often steeply inclined. Close to one of the mounds, 



