FOSSIL REEFS: DEVONIC 429 



On the western side of the State of Michigan, in the Petoskey 

 and Traverse Bay regions, at present only the marginal portions of 

 the reefs are shown in the outcrops and cuttings ; here the rock is 

 made up of fragments of Acervularia davidsoni, Favosites and 

 stromatoporoids embedded in the crystalline sand, these often form- 

 ing a veritable coral conglomerate, such as is found on the flanks of 

 modern coral reefs. Farther away from the reef the coral frag- 

 ments cease, once the rock becomes a well-stratified calcarenyte or 

 calcilutyte. That the waters in which these beds were deposited 

 were shallow is shown by the occasional cross-bedding and the ripple 

 marks found in these strata, as well as by the repeated phenomenon 

 of contemporaneous erosion and deposition, and the wedging out of 

 certain strata and the thickening of others. Not infrequently 

 seams of carbonaceous material separate some of the layers of the 

 limestone, and in these layers plant remains are not uncommon. 



The sections exposed below the city of Petoskey in the Traverse 

 Bay region are very instructive in this connection. As stated, they 

 represent the marginal portions of the reefs and show a succession 

 of erosion planes dividing the entire mass into strata from a few 

 feet to 10 or 20 feet in thickness. These strata show successive 

 accumulations of coral sand with worn pebbles of corals or stroma- 

 toporoids, generally of the size of a man's fist or larger, and ar- 

 ranged in strata which in some cases are horizontal, in others dip 

 in various directions at low angles. There is a certain succession 

 of remains in the various strata, those of the higher beds being dis- 

 tinct from those of the lower. In the upper beds the branching 

 stromatoporoid Idiostroiiia ccrspitosuiii is the chief organism, and 

 this is often found in the position of growth, its finger-like branches 

 spreading to capture the coral sand. Most of this coral sand has 

 since been dolomitized v/ith the result that the mass is rendered 

 porous. The coral pebble strata are often traceable for miles with- 

 out apparent change in character. 



These successive erosion planes suggest that periodic subsi- 

 dence was going on here during the growth of the reef, each stratum 

 being eroded when its growth 'had brought it to within the level 

 of wave activity. The change in fauna with the change in the 

 growth of the reef is analogous to the changes in fauna found at 

 successive steps in the building of modern reefs. A striking fea- 

 ture of the clastic strata of this region is the frequent intercalation 

 of calcilutytes made from the finest lime flour. These and the 

 equally pure beds of calcarenyte are in some cases separated by 

 layers of carbonaceous shales indicating a period of plant growth, 

 and probably even local terrestrial conditions in the form of vege- 



