THE SILURIAN PERIOD. 383 



evidence that some of the limestone after having once been solidified 

 was broken up by the waves and re-deposited, forming a coarse, brec- 

 ciated mass. In such cases the spaces between the fragments are often 

 filled with fine detritus in which fossil shells sometimes occur, as though 

 the shell-bearing animals had sought the protection of these sheltered 

 nooks, or as if their shells had been driven thither by the waves. 

 Evidences of wave-worn hollows and recesses in the reefs may also be 

 found, showing that waves were then wearing, as now, the summits of 

 rock-masses which were so slightly submerged as to come within the 

 limits of their action. 



The deposits between and about the ancient Wisconsin reefs are 

 similar ly instructive. On the sides and at the bases there accumulated 

 a mixture of shell and coral fragments and calcareous sands growing 

 finer and finer as the distance from the parent reef increased, and the 

 slope of the bottom became more gentle, until at a distance, having 

 passed through the several granular stages, there was a gradation into 

 white calcareous mud, which spread itself widely over the sea-bottom 

 around the reefs and between them and the adjacent land. This fine 

 calcareous sediment ultimately gave rise to the compact white lime- 

 stone of the region. 



On the submarine plains immediately bordering the reefs flourished 

 many forms of life which secrete lime carbonate. Mollusks and crinoids 

 seem to have been more abundant here than upon the reefs themselves. 

 Over the white mud-flats beyond there seems to have been a relative 

 paucity of life, the reef -frequenting forms being especially rare. 



In the ancient Niagara seas, therefore, at least four phases of lime- 

 stone were in process of accumulation simultaneously, viz.: (1) that 

 composed of coral which still remains unbroken; (2) that composed of 

 masses of coral partly disrupted; (3) that made up of the granular 

 products of shell and coral grinding; and (4) that made up of the still 

 finer products of comminution, all derived from essentially the same 

 source, the secretions of the reef life. To this category should perhaps 

 be added (5) the brecciated limestone, made up of the broken and re- 

 cemented fragments of beds which had been disrupted, and (6) the 

 oolitic limestone which occurs at some points in this and many other 

 limestone formations. The especial feature of this variety of limestone 

 (Fig. 179) is that it is made of tiny concretions of lime carbonate pre- 

 cipitated from solution about nuclei of any sort. Recent observa- 



