EVOLUTIONAL GEOLOGY. 307 



rocks were not buried under the full succession is more than probable. 

 However, they must have been buried to a great depth— at least sev- 

 eral miles— and thus subjected to high pressure and temperature, not- 

 withstanding which they are comparatively unaltered.'' ' 



I select this example because it is one of the best instances of a diffi- 

 culty that occurs more than once in considering the histor}^ of sedi- 

 mentary rocks. On the supposition that the rate of increment of tem- 

 perature 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 the Penokee rocks would attain a tempera- 

 ture of nearly SSS*^ C, and since water begms to exert powerful chem- 

 ical action at 180° 0. the}^ should, on the theory of a solid cooling 

 globe, have suffered a metamorphosis sufficient to obscure their resem- 

 blance to sedimentary rocks. Either, then, the accepted rate of down- 

 ward 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 5(),000 feet. Let us consider each alternative, and in the 

 tir.st place let us apply the rate of temperature increment determined 

 b}^ Professor Agassiz in this very Lake Superior district. It is 1° C. 

 for every 402 feet, and twenty-five millions of yenvs ago, or about the 

 time when we may suppose the Penokee rocks were being formed, it 

 would be 1° C. for everj^ 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 temperatui-e increment would meet the difficulty; but, on 

 the other hand, it would involve a period of several hundreds of mil- 

 lions of years for the age of the " consistentior status," and thus 

 greatlv exceed Professor ,Ioly's maxinmm estimate of the age of oceans. 

 We ma}^ therefore turn to the second alternative. As regards this it 

 is by no means certain that the 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 S3^stem that lies exposed to 

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

 seem impossible 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 held for investigation. 



THE FORMATION OF MOUNTAIN RANGES. 



It is frequently alleged that mountain chains can not be explained 

 on the hypothesis of a solid earth cooling under the conditions and for 

 the period we have supposed. This is a question well worthy of con- 

 sideration, and we may first endeavor to picture to ourselves the con- 

 ditions under which mountain chains arise. The floor of the ocean lies 



1 Tenth Animal Report U. S. Geolofiieal Survey, 1888-89, p. 457. 



