No, 3.] SPENCER — PALiEOZOIC GEOLOGY. 157 



Another conspicuous epoch in the history of the ancient sea 

 is ni;irked by tlie great bed of dolomite (No. 8 of section). At 

 no time was the sea so free from the influx of mechanical sedi- 

 ments. This bed with a thickness of about five feet forms an 

 enduring monument for the myriads of crinoids whose remains 

 most largely led to its formation, although subsequently it has 

 absorbed magnesia, which in the re-crystallization of its mole- 

 cules has obliterated all but the fragments of the oriiiinal se<r- 

 ments of their stems. 



Another noticeable change in the rock-making organisms is 

 found in a bed of dolomitic rock two and a half feet thick, almost 

 literally filled with the remains of three or four species of Strom- 

 at(ypora. This stratum is near the surface bed at Carpenter's 

 Limekilns. (Range VI, lot 15 of Barton) about three miles south 

 of the centre of the city of Hamilton. 



Besides the remains of life, as shown in these few more con- 

 spicuous beds, we find throughout the whole Niagara epoch that 

 Bryozoons were numerous; Crinoids were abundant (in places, 

 as at Grimsby, where some of the beds consist simply of masses 

 of these stems). Corals were dominant in some localities, and 

 Moilusks of every class were largely represented. 



The Niagara limestones have been largely derived from broken 

 shells, corals and other calcareous organisms, but subsequently 

 the calcareous matter has combined with, or a portion of it has 

 been replaced by, magnesia which had been precipitated amongst 

 the comminuted organisms. 



Henry C. Sorby, Esq., F.R.S., President of the Geological 

 Society of London, (Q.J.G.S., May, 1879,) has shown that the 

 condition in whicli calcareous matter is present in the structure 

 of shells, or of allied forms of life, has much to do with the sub- 

 sequent preservation of their remains in the rock, on the crystal- 

 lization of their particles into solid limestones. 



The principal condition in which lime is present in calcareous 

 organisms is as the carbonate, either in the form of calcite or 

 aragonite. However, there are some structures like the Lingula, 

 where the lime occurs, as the phosphate, the same as in bones. 

 The phosphate of lime is less apt to change its molecular condi- 

 tion than the caibonate, and, as a result, the shells of that 

 material, or parti illy of it, are generally better preserved in the 

 fossil condition than those of the carbonate. But these phos- 

 phatic shells have not contributed to any extent in the formation 

 of the Niagara limestones. 



