428 PRINCIPLES OF STRATIGRAPHY 



for example, the dip was 28°, then rapidly changed to 14°, and then 

 more gradually to 2°. The sand here is finer than that filling the 

 cavities in the reef, but it often includes fragments and partly worn 

 heads of Favosites, Stromatopora, etc., lying in all positions. Occa- 

 sionally the material is a, coral breccia, but for the most part it is 

 a calcarenyte. Farther away from the reef-mound it frequently 

 becomes a calcilutyte or rock made up of the finest lime flour. 

 Throughout the marginal zone there is an interlocking of organically 

 formed and fragmental lime rock, indicating a periodic spreading 

 outward of the reef and a subsequent overwhelming of each ex- 

 panded rim by fragmental deposits. These spreading fringes of the 

 reef consist mainly of the smaller branching corals and of bryozo- 

 ans, which at times extend far out on the foundation of coral 

 sand. These expanded rims, formed when erosion temporarily 

 ceased, extend outward to form the dividing planes between the 

 successive strata of the bedded limestones, where they are often 

 replaced by films of silty or even clayey material. These periodic 

 cessations of erosive activity on the reef and the spreading of the 

 reef fauna over the surrounding beds of unfossiliferous coral sand 

 must be due to the local protection of the reef mass from the 

 waves. This may be brought about by the becoming effective of 

 some outer barrier formed by the slow growth of an outer reef 

 which has become of sufficient height to break the force of the 

 waves, or by the building of a temporary sand or shell bar. At 

 such times the various minor organisms of the reef fauna find it 

 possible to spread in all directions in the relatively quiet water, and 

 so form a layer of fossiliferous material between the thicker layers 

 of coral sand limestone. It is on these dividing planes, as every 

 collector knows, that the largest number of specimens is found, the 

 limestones themselves only rarely carrying unworn fossils. 



In some cases, however, the dividing planes close to the reef 

 mark a period of especial destructiveness, when the smaller corals 

 and other organisms torn from the reef are rolled outward and 

 come to rest in more or less worn conditions as a bed of organic 

 conglomerate upon the uniformly grained clastic limestones. In 

 some cases the spreading organic rims are marked by the death of 

 the corals, especially the simple types, and by their partial solu- 

 tion. The cyathophylloid corals often have their epithecas dis- 

 solved away. On such a partly corroded surface Bryozoa and other 

 attached organisms grow, showing that conditions unfavorable to 

 the true corals did not affect some of the other organisms. These 

 conditions are well shown in the upper part of the reefs near 

 Alpena. 



