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As soon as our earth ceased to be incandescent, and became covered in large part by water, it commenced its depositions of submarine sediments. The oldest known sedimentary rocks, comprising the Laurentian and Huronian systems of Canada, have a total average thickness which cannot certainly be estimated at anything less than 30,000 feet. Sir William Logan, indeed, the greatest authority upon these primeval formations, considered the measurable thickness of his Upper and Lower Laurentian alone to amount respectively to 20,000 and 10,000 feet, while he set down the Huronian system as reaching some 18,000 more. But as doubts have been raised whether the Huronian series are not really the metamorphosed representatives of the Upper Laurentian, we will omit them altogether from our calculation, so as to avoid any possible cause of offense. The great Cambrian system, the next in order of time, has a thickness which has been fairly estimated at from 25 000 to 30,000 feet. We will adopt the smaller figure. The Silurian is pretty certainly known to number 6,000 feet. The Old Red Sandstone, with its doubtful contemporary, the Devonian, cannot be put down for less than 10,000. The Carboniferous series amount to at least 12,000 feet, the coal measures alone sometimes attaining to fully that thickness. Thus the whole Primary group, including the so-called azoic rocks, has a total vertical extent of not less than 83,000 feet. By the side of these enormous thicknesses, we can only allow 10.000 feet for the whole of the Secondary formation, from the Permian to the Chalk inclusive, while we shall be generous if we assign 1,000 feet to the little group of the Tertiary and Post-Tertiary deposits. This gives us a total thickness for the whole geological series of 94,000 feet. Let us allow 6,000 morecfor the breaks between each of these main divisions, or the unrepresented strata, and we have the round number 100,000 feet. A tabular statement will make these relations clear, and will allow us to translate our known thicknesses into conjectural but relatively ascertained dates: Feet. Years. Laurentian 30,000 30,000,000 Cambrian 25,000 25,000,000 Silurian 6,000 6,000,000 Old Red Sandst°ne ) ^00 10,000,000 Devonian ) ' ' Carboniferous 12,000 12,000,000 Secondary 10,000 10,000,000 Tertiary and Post-Tertiary 1,000 1,000,000 Gaps and unrepresented strata 6,000 6,000,000 100,000 100,000,000 —Gen Ueman's Magazine.
