724 REPORT— 1900. 



Penokee focks x^ere then buried to a great depth, the eiact atndunt depending 

 upon their horizon and upon the stage in Keweenaw time, when the tilting and 

 erosion, which brought them to the surface, commenced. 



' That the synclinal trough of Lake Superior began to form before the end of the 

 Keweenaw period, and consequently that the Penokee rocks were not buried under 

 the full succession, is more than probable. However, they must have been buried to a 

 great depth — at least several miles — and thus subjected to high pressure and 

 temperature, 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 60,000 feet the Penokee rocks would attain 

 a temperature of nearly 3.3.3° 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 temperature is erroneous, 

 or the Penokee rocks were never depressed, in the place where they are exposed 

 to observation, 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 deter- 

 mined by Professor Agassiz in this very Lake Superior district : it is 1° 0. 

 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 ieet, with a resulting temperature at a depth of 50,000 feet of 163° C. only. 

 Thus the admission of a very low rate of temperature 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 estimate of the age of the oceans. We may therefore 

 turn to the second alternative. As regards this it is by no means certain that tiie 

 exposed portion of the Penokee series ever was depressed 50,000 feet : the beds lie 

 in a synclinal the base of which indeed may have .sunk to this extent, and entered 

 a region of metamorphosis ; but the only part of the system that lies exposed to 

 view is the upturned margin of the synclinal, and as to this it would seem impos- 

 sible to make any positive assertion as to the depth to which it may or may not have 

 been depressed. To keep an open mind on the question seems our only course for 

 the present, but difficulties like this offer a promising field for investigation. 



The Formation of Mountain Ranges. 



It is frequently alleged that mountain chains cannot be explained on the hypo- 

 thesis of a solid earth cooling under the conditions and for the period we have 

 supposed. This is a question well worthy of consideration, and we may first 

 end>-avour to picture to ourselves the conditions under which mountain chains arise. 

 The floor of the ocean lies at an average depth of 2,000 fathoms below the land, 

 and is maintained at a constant temperature, closely approaching 0° C, by the 

 passage over it of cold water creeping from the polar regions. The average tem- 

 perature of the surface of the land is above zero, but we can aHbrd to disregard the 

 difference in temperature between it and the ocean floor, and may take them both 

 at zero. Consider next the increase of temperature with descent, which occurs 

 beneath the continents : at a depth of 13,000 feet, or at same depth as the ocean 

 floor, a temperature of 87° C. will be reached on the supposition that the rate of 

 increase is 1° C. for 150 feet, while with the usually accepted rate of 1° C. for 

 108 feet it would be 120° C. But at this depth the ocean floor, which is on the 

 same spherical surface, is at 0° C. Thus surfaces of equal temperature within the 

 earth's crust will not be spherical, but will rise or fall beneath an imaginary 

 spherical or spheroidal surface according as they occur beneath the continents or the 

 oceans. No doubt at some depth within the earth the departure of isothermal 



« Tenth Annual EepoH U.S. Geol. Survey, 1888-89, p. 457. 



