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from physical considerations, more especially by Sir William Thomson, which limit the possible existence of the earth's solid crust to one hundred millions of years. Similar conclusions have also been deduced from what is known of the physical constitution of the sun. Croll's own ingenious theory of glacial periods produced by the varying eccentricity of the earth's orbit, along with the precession of the equinoxes, would give, according to him, about 80,000 years ago for the date of the Glacial period, and for the beginning of the Tertiary period about 3,000,000 years ago. It would thus appear that physical and geological science conspire in assigning a great antiquity to the earth, but not an unlimited antiquity. They agree in restricting the ages that have elapsed since the introduction of life within one hundred millions of years. I confess, however, that a consideration of the fact that all our geological measures of erosion and deposition seem to be based on cases which refer to what may be termed minimum action leads me to believe that the actual time will fall very far within this limit. For example, if we were to suppose an elevation of the land drained by the Mississippi even to a small amount, its cutting power would be vastly increased for a long time. The same effect would result from a subsidence and re-elevation, or from any cause increasing the amount of rainfall or deposition of snows in winter. Now we know that such things have occurred in the past, while we have no reason to believe that the amount of action was ever much less than at present. Similar considerations apply to nearly all our geological measures of time; and there has been a tendency to exaggerate these, as if geologists were entitled to demand unlimited time, and to stretch the doctrine of uniformity to the utmost. 6. During the whole time referred to by geology, the great laws both of inorganic and organic nature have been the same as at present. The evidence of light and darkness, of sunshine and shower, of summer and winter, and of all the known igneous and aqueous causes of change, extends back almost, and in some of these cases altogether, to the beginning of the Palaeozoic period. In like manner the animals and plants of the oldest rocks are constructed on the same physiological and anatomical principles with existing tribes, and they can be arranged in the same genera, orders, or classes, though specifically distinct. The re
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