THE SILURIAN PERIOD 541 



no preceding formation of limestone except the Trenton is so wide- 

 spread. It is the oldest formation in which well-developed coral 

 reefs .have been identified, though coral-secreting polyps had lived 

 before (p. '532) . The reefs are known in eastern Wisconsin, Indiana, 

 and elsewhere. 



These reefs and the deposits adjacent to them illustrate as clearly 

 as anything among the ancient formations, the origin of several 

 varieties of limestone. The reefs themselves are composed of the 

 commingled relics of the life that grew upon them. Great masses 

 of coral sometimes stand erect in the rock just as they grew, having 

 escaped destruction during their burial in the growing reef. In 

 other instances, the coral masses in the limestone are fragmentary, 

 broken and worn by the waves. With the larger pieces of coral 



Fig. 398. The Wabash dome in the Niagara limestone. (Kindle.) 



there is coarse and fine detritus, the product of coral comminution. 

 These combine to make up the mass of the reef-rock. Between and 

 about the reefs a mixture of shell and coral fragments and calcareous 

 sands accumulated, growing finer as distance from the parent reef 

 increased and the slope of the bottom became more gentle, these 

 materials grading finally into calcareous mud, which was spread 

 widely over the sea-bottom about the reefs. This fine calcareous 

 sediment ultimately gave rise to compact white limestone. 



Thickness and structure. Unlike the preceding formations of 

 the Silurian, the Niagara is not thicker in the east than elsewhere. 

 In the east, indeed, where the formation is exposed, it has a thick- 

 ness of but 100 to 300 feet, while in Wisconsin it attains a maximum 

 of 800 feet (perhaps including some Clinton), all of which is lime- 

 stone. While the Niagaran beds of the interior are in general nearly 

 horizontal, they are frequently domed so as to give the beds a high 

 angle of dip (Fig. 398) . This is true, for example, at various points 

 about the southern end of Lake Michigan. 



