790 



SCIENCE. 



[N. S. Vol, XII. No. 308. 



by metamorphosis. The series itself is 

 14,000 feet thick. It was covered before 

 being upturned with a great thickness of 

 Keweenaw rock. This series of the Mon- 

 treal Eiver is estimated to be 50,000 feet 

 thick. Adding to this the known thickness 

 of the Penokee series, we have a thickness 

 of 64,000 feet. * * * The Penokee rocks 

 were then buried to a great depth, the 

 exact amount 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 

 thejr are comparatively unaltered." * 



I select this example because it is one of 

 the best instances of a dif&cultj' 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 the Penokee 

 rocks would attain a temperature of nearlj' 

 333° C; and since water begins to exert pow- 

 erful chemical action at 180° C. they should, 

 on the theory of a solid cooling globe, have 

 suffered a metamorphosis suflScient to ob- 

 scure their resemblance to sedimentary 

 rocks. Either then the accepted rate of 

 downward increase of temperature is erro- 

 neous, or the Penokee rocks were never de- 

 pressed, in the place where they are exposed 

 to observation, to a depth of 50,000 feet. 

 Let us consider each alternative, and in 



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

 p. 457. 



the first place Jet us apply the rate of tem- 

 perature 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 305.6 feet, with a resulting tempera- 

 ture, 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 ' con- 

 sistentior 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 the exposed 

 portion of the Penokee series ever was de- 

 pressed 50,000 feet ; the beds lie in a syn- 

 clinal 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 up- 

 turned 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 oflfer a promising field 

 for investigation. 



THE FORMATION OF MOUNTAIN RANGES. 



It is frequently alleged that mountain 

 chains cannot be explained on the hypoth- 

 esis of a solid earth cooling under the 

 conditions and for the period we have sup- 

 posed. This is a question well worthy of 

 consideration, and we may first endeavor 

 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 approach- 



