GEOLOGY. 233 



causes the isogeothcrmal lines steadily to rise. The laying down 

 of one hundred thousand feet of strata would bring a temperature 

 of about 1,700° into the lowest part of the beds which were formed 

 in water having the ordhiary temperature of the sea. This must 

 cause a great lateral strain in the lower part of the section be- 

 neath the sea floor where strata are being accumulated, and as 

 tiiere is no such lateral strain in the upper part of the section the 

 result will be, necessarily, to cause the crust beneath such an area 

 of deposition to tend to bend downwards. The result in this case 

 is comparable to what occurs when two metals having different 

 coefficients of expansion are soldered together and then subjected 

 to the action of heat. The compound bar tends to arch in the di- 

 rection of the metal which expands the most. In the section of 

 the crust beneath the sea, the lower part, which expands the most, 

 is also on the outside of the curve. Even if the whole crust sec- 

 tion beneath the areas of deposition expanded equally, the ten- 

 dency of the strain produced would still be to cause the actual 

 curve of the sea floor to be deepened whenever the crust came 

 to contract further in order to readjust itself to the diminished 

 nucleus. 



The frequent submergence of parts of the continents after they 

 had been lifted above the sea does not conflict with this theory ; 

 the alteration of the pivot point of the movement would go far to 

 account for these changes.* 



The truth of these conclusions does not depend upon the inter- 

 nal condition of the earth in any way, except that the mass is 

 supposed to be intensely heated. It may be either fluid or essen- 

 tially solid without aff"ecting these conclusions. — Abstract of View 

 set forth in Lectures at Harvard University in 1864 and 1869, hy N.- 

 S. Shaler, 



GEOLOGY* 



The question, How far the variation of the eccentricity of the 

 earth's orbit may have brought about the great changes of climate 

 indicated by geological phenomena, has been often discussed, 

 more especially as regards the cause and date of glacial epochs. 

 During the past three millions of years there have been three pe- 

 riods when the eccentricity attained a high value. The first of 

 tliese began about 2,630,000 years ago, and terminated about 

 2,460,000 years ago. The second began about 980,000 years ago, 

 and terminated about 720,000 years ago. The third began about 

 240,000 years ago, and terminated about 80,000 years ago. The 

 , third period, Mr. Croll considers, was the date of the glacial 

 epoch ; the second was that of the upper miocene period ; while 

 the third corresponded to the glacial epoch of the middle eocene 

 period. Few geologists believe that during the two latter periods 

 our country passed through conditions of glaeiation as severe as it 

 has done during the post-pliocene period. Mr. Croll, however, 

 argues that subaerial denudation, by destroying the whole of 



* See abstract of paper on the changes of level of seashores, p. — . 

 20* 



