30 THE AGE OF THE EARTH 



ture, notwithstanding which they are comparatively 

 unaltered." * 



I select this example because it is one of the best 

 instances of a difficulty that occurs more than once in 

 considering the history of sedimentary rocks. On the 

 supposition that the rate of increment of temperature 

 with descent is 1 F. for every 84 feet, or 1 C. for every 

 150 feet, and that it was no greater during these early 

 Penokee times, then at a depth of 50,000 feet t the 

 Penokee rocks would attain a temperature of nearly 333 

 C. ; and since water begins to exert powerful chemical 

 action at 180 C., they should, on the theory of a solid 

 cooling globe, have suffered a metamorphosis sufficient to 

 obscure their resemblance to sedimentary rocks. Either, 

 then, the accepted rate of downward increase of tem- 

 perature is erroneous, or the Penokee rocks were never 

 depressed, in the place where they are exposed to observa- 

 tion, to a depth of 50,000 feet. Let us consider each 

 alternative, and in the first place let us apply the rate of 

 temperature increment determined by Professor Agassiz 

 in this very Lake Superior district : it is 1 C. for every 

 402 feet, | and twenty-five millions of years ago, or about 

 the time when we may suppose the Penokee rocks were 

 being formed, it would be 1 C, for every 305 '5 feet, with 

 a resulting temperature at a depth of 50,000 feet of 163 C. 

 only. Thus the admission of a very low rate of tempera- 

 ture increment would meet the difficulty; but on the 

 other hand it would involve a period of several hundreds 

 of millions of years for the age of the " consistentior 

 status," and thus greatly exceed Professor Joly's maximum 



* " Tenth Annual Report U.S. Geol. Survey," 1888-89, p. 457. 



f From information received from Mr. A. C. Lane, State Geologist 

 of Michigan, 30,000 feet would be nearer the mark. On the other 

 hand the rise of temperature is now estimated at 1 C. for 193 feet. 



t We have already seen that this rate is far too low. 



