HAMILTON REEFS OF ALPEXA, MICHIGAN 339 



On the sides of the reef the bedded strata, which consist chiefly of con- 

 solidated crinoid and coral sand, dip away at high ano^les. Measure- 

 ments in one of the quarries in Alpena (Collins') showed an inclination 

 of 28 deojrees near the reef, falling rapidly to 14 degrees and then more 

 gradually to 2 degrees, which is the normal dip of the strata. Through- 

 out the marginal zone there is an interlocking of organicall}' formed and 

 fragmental lime rock, indicating a periodic spreading outw^ard of the 

 reef and a subsequent overwhelming of each expanded rim by fragmental 

 deposits. These spreading fringes of the reef consist mainly of the 

 smaller branching corals and of br^'ozoa, which at times extended far 

 out on the foundation of coral sand. In fact, it is these expanded rims? 

 formed when erosion temporarily ceased and when only fine silt was 



Figure I.— Diagrammatic Cross-section of a Divonic Coral Reff. 

 From Traverse group, Alpena, Michigan, showing central reef portion of coral heads, with 

 coral-sand filling, and bedded clastic limestones (calcarenites, etcetera) surrounding it and dip- 

 ping away from it. At intervals the reef spreads out, dividing the bedded limestone. 



deposited, that mark the division of the fragmental limestone into tiers 

 or beds. The beds themselves, often several feet thick, are generally 

 un fossil iferous, though here and there a more or less perfecth^ preserved 

 fossil may be found. For the most part, however, organic remains, more 

 or less fineh^ ground up, make up the beds of limestone. They are, 

 however, commonly separated by thin shaly films, which occasionally 

 ma}^ have a perceptible thickness. It is during the formation of these 

 separating layers, which mark the periods of non-deposition of the frag- 

 mental lime sand, that the organisms of the reefs creep outward over the 

 stratified beds, and it is here, as every collector knows, that fossils must 

 be looked for. 



Chemical analyses of the rock show that the purest limestone is found 

 in the center of the reef, where the per cent of CaCOo is not infrequently 

 over 99. 



REEFS OF THE TRA VERSE BA Y REGIOX 



Xo actual exposures of reefs occur on Little Traverse bay, but in the 

 cliffs between Petoskey and Bay Shore a number of sections of marginal 

 zones of reefs are furnished. In these sections the rock is chieflv a 



