4i8 PRINCIPLES OF STRATIGRAPHY 



ness. (Walcott-89, pi. II.) It also occurs in similar beds in 

 Alberta and British Columbia, and in Arizona. 



Ordovicic. Reefs of the bedded type have been reported from 

 the Lower Ordovicic, Cryptozoon minnesotcnsis forming the chief 

 reef builder. Little is known about the detail of occurrence, thick- 

 ness, and extent of these reef masses, which have been found in 

 many localities in the eastern United States and Canada. In the 

 Middle Ordovicic (Chazyan) the genera Stroniatoceriitm and Sty- 

 larcca form similar developments in New York, Tennessee, Ken- 

 tucky and Oklahoma. In the Black River horizon Stromatocerium 

 is reen forced by Columnaria and Tetradium, in Tennessee and 

 Kentucky, and these genera also form reef-like associations in the 

 Cincinnatian of these states. These same genera, together with 

 Beatricia, Labechia, Calapoecia, etc., form reef -like associations in 

 the Richmond of North America from Baffin Land and Anticosti 

 to the Mississippi Valley and also in Colorado, Nevada, Wyoming 

 and Alaska. ( Vaughan-87 '.244.) None of these is to be regarded as 

 a. typical reef, these structures being rather of the type of plate-like 

 expansions or bedded reefs, often of not more than a single layer 

 of hydrocorallines. They were undoubtedly an important source 

 of the lime sands and lime muds of the formations in which they 

 occur, though in many cases they seem to lie in undisturbed position 

 as of growth. It is not improbable, however, that the corals and 

 hydrocorallines of one section were grovmd up to form the sand 

 which covers undisturbed layers of the same horizon. 



SiLURic. Coral reefs are well developed in the Siluric, where 

 typical Palaeozoic reef builders — Favosites, Heliolites, Halysites, 

 Syringopora, etc. — among the corals, and stromatoporoids among 

 the hydorcorallines abound. They have been especially described 

 from Wisconsin and from the Island of Gotland in the Baltic Sea. 

 Others occur in the Upper Siluric of western Ontario and Michi- 

 gan, in the Louisville limestone of the Falls of the Ohio, the Lewis- 

 town limestone of Pennsylvania, and in the Wenlock of England 

 and in Bohemia. 



Niagaran Reefs of Wisconsin. The Siluric strata of south- 

 eastern Wisconsin are characterized by typical coral reefs which 

 form mounds almost entirely composed of coral and Stromatopora 

 masses, and separated from each other by interspaces of some 

 extent, in which the coral sand and mud were deposited in stratified 

 layers. (Chamberlin-15 :?d^; Grabau-40.) These mounds stand 

 out in relief, owing to the peculiar resistance of the structureless 

 reef rock to erosion, while the bedded strata of calcarenyte and 

 calcilutyte between them more easily suffered denudation. In the 



