280,000,000 years ago the continents had largely emerged from the second Silurian inundation. The rock deposits of this submergence are known in North America as Niagara limestone. Several layers extend over Canada.  260,000,000 years ago North America was partially overspread by seas that in Canada and other areas leave coral deposits.  In Canada there are seventy-five layers of coal in the various beds, which indicate the number of times the land fell and rose.