216 Fisher — Cause of Changes of Levelin the Earth's Crust. 



Art. XV. — A Suggested Cause of Changes of Level in the 

 Earth's Crust; by Rev. O. Fisher. M.A., F.G.S., Cam- 

 bridge, England ; Author of the " Physics of the Earth's 

 Crust." 



Dr. Spencer in America,* and Prof. Hull in England, f have 

 for some years past been drawing the attention of. geologists 

 to the great changes in the relative levels of land and ocean 

 on both sides of the Atlantic, evidenced by the drowned 

 plains bordering the continents intersected by deep canyons, 

 which are the now submerged continuations of existing river 

 channels. These changes of relative levels amount frequently 

 to a mile or more. 



Great changes of level are among the rudiments of the 

 grammar of G-eology. But the recency of those described, 

 viz. Pliocene or early Pleistocene times, is remarkable. 



It rests with those who believe that the earth is solid to 

 explain these phenomena on that hypothesis. But for those 

 geologists who think that a liquid substratum of unknown 

 depth underlies the cooled crust, the following explanation 

 appears plausible. 



It being granted that the earth is a cooling body, if the 

 cooling takes place in a liquid which expands by heat it will 

 do so by couvection currents. Hence convection currents 

 must exist in the substratum. The action of such currents 

 has not yet been brought under the dominion of mathematical 

 methods, but the general principle of their action appears to 

 be as follows : Suppose a mass of liquid consisting of parallel 

 layers increasing in temperature with the depth. It would be 

 in unstable equilibrium, and that state could not continue; 

 somewhere a column of the fluid would begin to ascend, and 

 to flow away horizontally at the surface. The waste in the 

 lower parts of the ascending column would be supplied by the 

 inflow of the surrounding liquid, which would depress the 

 isotherms in it, and the whole of the region affected would 

 become cooler. But momentum will have been gained in the 



* Reconstruction of the Antillean Continent, Bull. Geol. Soc. America, 

 vol. v, p. 103, 1895. Great Changes of Level in Mexico, Bull. Geol. Soc. 

 Arner. vol. ix, p. 13, 1898. Submarine Valleys off Amer. Coast and in the 

 North Atlantic, lb. vol. xiv, p. 207, 1903. The Submarine Great Canon of 

 the Hudson River, this Journal, vol. xix, p. 1. 1905. Physiographic Improb- 

 ability of Land at the North Pole and Bibliography of Submarine Valleys of 

 North Amer., lb. p. 333, 1905. Reply to Mr. Huddleston, Geol. Mag., vol. 

 xi, p. 559, 1899. 



f Submerged Terraces and River Vallevs bordering the British Isles, Trans. 

 Victoria Institute 1897-8; also Geol. Mag., vol. x, p. 351, 1898. The Sub- 

 merged Platform of Western Europe. Geol. Mag., p. 478, 1898. Suboceanic 

 Physical Features, Geol. Mag., vol. vi, p. 132, 1899. 



