458 Notices of Memoirs — Professor Sollas's Address 



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 50,000 feet 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 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 determined 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 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 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 the exposed portion of the Penokee 

 series ever was depressed 60,000 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 a& 

 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 oifer a promising field for 

 investigation. 



The Formation of Mountain Banges. 

 It is frequently alleged that mountain chains cannot 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 consideration, and we may first endeavour 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 temperature of the surface of the land is above zero, but 

 we can afford to disregard the difference in temperature between it 



1 Tenth Annual Eeport U.S. Geol. Survey, 1888-89, p. 467. 



