HuTTON. — On the Formation of Mountains. xxix 



perature by radiation V 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 experiments 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 con- 

 ductivity 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 formation 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 

 oOO feet in thickness ; but a deposit 500 feet in thickness imj^lies 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 j)robable 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 conduction 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 palaeozoic 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 half millions of 

 years ago. From that time to the present, elevation must have gone on in an 

 increasing scale ; but, although increasing in height, it must have also been 

 decreasing in rapidity, and a time must inevitably arrive when elevation will 

 be so slow that it will do no more than equal denudation, and when again, 



* Theorie de la Chaleur, Paris, 1 822. 



