I Used to think that the corrugations of the earth's crust were due to compression through the shrinking of the interior. To judge of the sufficiency of this cause the first thing to be done is to seek a measure of the compression, and then to compare the result of the effects of cooling with the actual amount of compression. The most satisfactory measure appears to be the thickness of the layer which the corrugations would form if levelled down. The question then becomes one of how much. In 1863 Lord Kelvin (then Sir W. Thomson) formulated a law of secular cooling upon the hypothesis that the interior is solid. Adopting a probable value for the contraction of rocks in cooling, I calculated the thickness of the layer which would be produced by the corrugations resulting, and found it far short of that which the existing inequalities would form if levelled down. Mr. Mellard Reade and Dr. Davison subsequently discovered the existence of a level of no strain within the crust, and this greatly reduces the possible amount of corrugations. The conclusion at which I arrived was that, on the hypothesis of a solid globe, secular contraction through cooling would not account for the corrugations.