288 On the Physical Geology of the United States, Sc. 
with the abundance of trappean and other unstratified rocks, the 
quartz and other veins, and their effects in producing metamor- 
phic changes on the adjacent masses, during the epochs of the 
earlier fossiliferous deposits. 
We may admit that the earth is in the state of a cooling body, 
far warmer in its mean temperature on its surface than the re- 
gions of space in which it moves around the sun, although long, 
very long periods of time make no appreciable difference in its 
mean temperature.* 
If the earth be a cooling body, it must have diminished in 
volume, in obedience to the law that bodies contract by diminished 
temperature, and expand by heat. 
If the earth has diminished in volume, it has increased in its 
velocity of rotation, (in obedience to a well known dynamical 
law.) This would tend to shorten the length of the day, and it 
has been shown by La Place, that for two thousand years at least 
the length of the day, or of a revolution of the earth on its axis, 
has not varied ;1, of a centesimal second.t 
It might here be said that this is sufficient evidence that the 
earth has not contracted, and that no geological effects have been 
produced, dependent on such a cause. 
It may be said in answer : 
1st. The rate of cooling and consequent contraction is extreme- 
ly slow, and long periods would be required to make it manifest. 
2d. The effects of the contraction in the production of currents 
in the ocean, and the sedimentary deposits from them, and the 
effects on the rocks themselves, lead to the conclusion that the 
changes of volume were parorysmal rather than secular; that 
a gradually accumulating tension was finally overcome by a 
sudden yielding of the solid strata, producing earthquakes and un- 
dulations of the surface, an increased velocity of rotation on the 
earth’s ‘axis, an increased flow of the equilibrating currents of 
* The Baron Fourier has given the following as the law of the diminishing tem- 
perature of the earth. The diminution is equal to the present mean temperature 
divided by double the number of centuries since the cooling process commenced. 
He concludes that since the time of the Greek school at Alexandria, the mean 
temperature has not varied from loss of radiant heat ‘sto° of the Centigrade (sto° 
Fahrenheit’s) thermometer—.4m. Jour. Sct. Vol. XXXII, p. 16. 
t This conclusion was deduced by calculating backwards the times of eclipses 
that were registered in Ptolemy’s Almagest, and observed in the time of Hip- 
parchus. 
