170 Notices of Memoirs — Sutton, on Mountains. 



rate of the conduction of heat outwards, no deposit would rise above 

 the surface of the sea, for as soon as deposition ceased the increase of 

 temperature would cease also ; and converselj'-, the greater the differ- 

 ence between the rates the greater would be the rise, for the longer 

 would be the time before the deposit attained its normal temperature. 



The data to estimate these rates are not very exact, more especially 

 the rate of deposition, but the following is the best information that 

 I can collect : — 



Monsieur Joseph Fourier has calculated that the earth decreases in 

 temperature by radiation 1° Fahr. in 3,000,000 years,^ and from this 

 we can deduce that the conduction outwards must be about one-tenth 

 of. an inch a year. Sir W. Thomson's calculations, founded on ex- 

 periments made at Edinburgh, Greenwich, and Upsala, give an 

 outward conduction two and three quarter times as fast ; but these 

 experiments were made on dry rocks, and he allows that if the rocks 

 were saturated with water, as all newly-formed deposits would be, 

 his estimate would have to be reduced by one-half; which would 

 then give an outward conduction of one-eighth of an inch per year. 

 Peclet's experiments show also that the conductivity of limestone is 

 only two-fifths of the average taken by Sir W. Thomson, or one-tenth 

 of an inch per year. Consequently, we cannot be far wrong if we take 

 the average conductivity outwards at one-ninth of an inch per year. 



Professor Dana has estimated that limestone grows at the rate of 

 one-eighth of an inch per year, and sandstone five to ten times as 

 fast, or from five-eighths to one and a quarter inch per year ; while 

 the average increase in thickness of the clays of deltas appears to be 

 about one-fifth of an inch per year ; so that if we suppose a forma- 

 tion to be about one-third limestone, we get an average rate of 

 deposition for the whole formation of one-third of an inch per year, 

 or three times as fast as the conduction of heat outwards. 



If at the present time the internal heat travels outwards at the 

 rate of one-ninth of an inch per year, it would take 54,000 years to 

 heat a deposit 500 feet in thickness ; but a deposit 500 feet in thick- 

 ness implies a rise of temperature in the underlying rocks of 10°, 

 which implies an elevation of 1,140 feet if the heated area was 100 

 miles in diameter, or 1,900 feet if it was 2,000 miles in diameter, 

 consequently in the first case the land would have risen 1,140 feet, 

 and in the last 1,900 feet in 54,000 years, or at the rate of from two 

 to three and a half feet per century, which is just the rate that Sir 

 C. Lyell considers as most probable from observation. 



But in former times when the internal temperature increased 

 three times as fast as it does now, or 1*^ for seventeen feet, the con- 

 duction outward would be equal to the deposition, and consequently 

 no land could rise above the water, and this may have been the cause 

 of the " insular condition " which seems to have prevailed over the 

 world during PalEeozoic times. According to the theory of the 

 secular cooling of the earth, advocated by Sir W. Thomson, these 

 conditions must have occurred about eleven and a half millions of 

 years after the formation of the crust, or about eighty-eight and a 

 1 Theorie de la Chaleur, Paris, 1822. 



