Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/161

Rh 2.4° C. for each 100 m. Since here, again, an abnormally slight gradient may be assumed on account of the convex form of the mountain, 2.5° C. per 100 m. can be considered as a good average value for the continental blocks. Corresponding measurements cannot naturally be carried out in the sima. If Friedländer’s statement is correct, which finds for hypabyssal igneous rocks, a smaller thermal conductivity and a temperature gradient of 6° C. for 100 m., then, by rectilinear extrapolation, at 9 km. depth in the continental block (beneath the sea-level), the same temperature (about 230° C.) would prevail as under the ocean. Beneath this level, however, the rocks below the ocean would be hotter than the strata at the same depth on the continental block. Friedländer’s figures are, of course, reliable only to a small extent. But a slight difference of thermal conductivity of this character is sufficient to compensate for the fact that on the floor of the ocean at 5 km. below sea-level a temperature of 0° C. prevails, whilst the continental blocks at the same depth have already a temperature of about 135° C.

By the use of linear extrapolation at a depth of 100 km. in the continental block, a temperature of 2500° C. is reached, a figure far above the melting-point of igneous rocks. It is universally agreed, however, that such a rectilinear extrapolation is inadmissible. But unfortunately we do not know the law by which the temperature alters with depth. Probably in the first place it is dependent on the