He estimates the average thickness of the sedimentary crust of the earth to be at least one mile, and from a computation of the proportion of carbonate and sulphate of lime to materials held in suspension in various river-waters from a variety of formations, infers that one-tenth of this crust is calcareous. He estimates the annual flow of water in all the great river-basins, the proportion of rain-water running off the granitic and trappean rocks, the percentage of lime in solution which they carry down, and arrives at the conclusion that the minimum time requisite for the elimination of the calcareous matter contained in the sedimentary crust of the earth, is at least six hundred millions of years. A writer in the _Gentleman's Magazine_[74] (Professor Huxley?), whose article I am only able to quote at second-hand, makes an estimate which, though much lower than the above, is still of enormous magnitude, as follows:-- Feet. Years. Laurentian 30,000 30,000,000 Cambrian 25,000 25,000,000 Silurian 6,000 6,000,000 Old Red and Devonian 10,000 10,000,000 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 ----------- Total 100,000,000 Mr. Darwin, arguing upon Sir W. Thompson's estimate of a minimum of ninety-eight and maximum of two hundred millions of years since the consolidation of the crust, and on Mr. Croll's estimate of sixty millions, as the time elapsed since the Cambrian period, considers that the latter is quite insufficient to permit of the many and great mutations of life which have certainly occurred since then. He judges from the small amount of organic change since the commencement of the glacial epoch, and adds that the previous one hundred and forty million years can hardly be considered as sufficient for the development of the varied forms of life which certainly existed towards the close of the Cambrian period. On the other hand, Mr. Croll considers that it is utterly impossible that the existing order of things, as regards our globe, can date so far back as anything like five hundred millions of years, and, starting with referring the commencement of the Glacial epoch to two hundred and fifty thousand years ago, allows fifteen millions since the beginning of the Eocene period, and sixty millions of years in all since the beginning of the Cambrian period. He bases his arguments on the limit to the age of the sun's heat as detailed by Sir William Thompson.