374 On some Results of the Earth's Contraction from Cooling, 



(7) That the locus of the region of subsidence on a conti- 

 nental border was in general along side of a region of thickly 

 stiffened unyielding continental crust, and that pressure against 

 the stable area beyond was one source of the catastrophe of 

 mountain-making. 



(8) That each epoch of plication and mountain-making ended 

 in annexing the region upturned, thickened and solidified, to 

 the stiffer part of the continental crust, and that consequently 

 the geosynclinal that was afterward in progress occupied a 

 parallel region more or less outside of the former, either land- 

 ward or seaward, and commonly the latter. 



(9) The principle adopted from LeConte, that the bottom of 

 a geosynclinal becomes weakened as subsidence and surface sedi- 

 mentary accumulations go forward, through the access of heat 

 from below or the rise of the isogeotherms (the change of level 

 in a given isothermal plane having been seven miles in the Appa- 

 lachian region), and that this in an important degree has made 

 possible the catastrophe from which synclinoria have resulted. 



(10) That while igneous eruptions and metamorphism have 

 each attended the formation of synclinoria, still in cases where 

 the plication was greatest the igneous eruptions have been least 

 in amount or absent ; and that the most extensive igneous erup- 

 tions have taken place on continental borders after the crust 

 had become too much stiffened to bend freely before the lateral 

 pressure. 



(11) That in the upturning and plication attending moun- 

 tain-making the heat from the transformation of the motion was 

 sufficient (in connexion with other heat from a rise of the iso- 

 geotherms due to previous surface -accumulations) to cause 

 metamorphism, and also the pasty fusion which obliterates all 

 stratification and gives origin to granite, and which may fill 

 cavities or fissures, and so make veins that have all the aspect of 

 true igneous ejections; and, as a more extreme effect, it may 

 produce, as Mallet says, the degree of fusion which belongs to 

 plastic trachyte, and give rise to trachytic and other ejections 

 through fissures or volcanic vents. But— 



(12) That the chief source of igneous rock is the plastic layer 

 situated beneath the true crust, or the local fire-seas derived from 

 that layer. 



The discussion has enlarged beyond its limits in my previous 

 publications; and many additional facts and conclusions have been 

 Drought forward. The various conclusions go forth to be tested 

 by the further developments of science. 



[Note. — The principle that the bottom of a geosynclinal be- 

 comes weakened as subsidence and surface- accumulation go 



