the Earth's Contraction from Cooling. 217 



A final word on Mountain-making \ — From the above we 

 learn that in the work of mountain-making in eastern North 

 America there was first the commencing and progressing 

 geanticlinal on the sea-border, and, as a concomitant effect 

 of the lateral pressure, a parallel geosynclinal further west 

 along the border of the continent. Concurrently the deepen- 

 ing trough of the geosynclinal was kept filled to the water- 

 level, or nearly, by sedimentary accumulations until these 

 had become seven miles in thickness; and, as a consequence, 

 the lines of equal temperature (isogeotherms) in the crust be- 

 neath gradually rose upward seven miles; and further, the geo- 

 synclinal crust, owing to this rising of heat from below, lost part 

 of its thickness by a melting-off of an under portion, and also 

 part of its strength up to a higher level by the softening action 

 of the heat, w r hile it received, as the only compensation for 

 the loss of thickness, the addition of half-consolidated sedi- 

 ments above. Finally, the geosynclinal region, owing to its po- 

 sition against the more stable continental mass beyond it, and 

 to the weakness produced in its crust in the manner explained, 

 became, under the continued lateral pressure and the gravity 

 of the geanticlinal, a scene of catastrophe and mountain-making 

 after the manner described. 



The principle here brought in, that the weakening of the 

 crust through the rise of the isogeotherms was one occasion^of 

 the catastrophe, is made of prominent importance by Professor 

 LeConte (Am. Journ, Sci. vol. iv. p. 468), though by a somewhat 

 different method. 



The geological facts thus far gathered have not yet proved 

 that there was a geanticlinal on the Pacific border (like that of 

 the Atlantic), as a counterpart to the geosynclinals in progress ; 

 but the evidence may be looked for with confidence. 



III. Metamorphism. 



The fact that all metamorphic or crystalline rocks are up- 

 turned or plicated rocks has led many to believe that disturbance 

 and plications were essential features of an epoch of metamor- 

 phism, and that HerschePs theory, which attributes metamor- 

 phism to the heat that rises into the strata owing to accumu- 

 lation above (a rise of the isogeotherms), is insufficient. This 

 conclusion is certainly confirmed by finding no evidence of meta- 

 morphism in the lowest beds of the Carboniferous series of Nova 

 Scotia, where, since the series has a thickness of nearly 16,000 

 feet, according to two of the best geologists in the world, Logan 

 and Dawson, the bottom temperature must have been, when the 

 series was completed, at least 330° F. It is still better sustained 



