64 ON MR. M. READE's WORK ON MOUNTAIN BUILDING. 



component materials into juxtaposition." This appears to 

 be the key to Mr. Mellard Reade's theory. 



Thus, in a given area such combinations may take place 

 as to produce chemical activity with increase of heat and 

 bulk, resulting in the upheaval of the overlying crust ; under 

 other conditions there will be a decrease of activity, with 

 such decrease of heat and bulk as will permit the overlying 

 crust to sink down and form a depression, greater or less in 

 depth and extent. 



In order to initiate a chain of great mountains, with its 

 core of archsean rock flanked by sedimentary strata, two 

 preliminary conditions are necessary. The one is, a deposit 

 of sediment on a subsiding area ; the other, that this area 

 must be limited, and the depression deep. 



Sediment, however, does not initiate subsidence, but its 

 accumulation sinks the crust to a point below that to which 

 its own weight alone would have carried it, nor will the 

 subsidence be counteracted by any great rise of the 

 Isogeotherms due to increased thickness of strata, these 

 being more largely influenced by the condition of the heated 

 magma below the area. 



In course of time diminished activity will be succeeded 

 b}^ a return to the normal state, and the Isogeotherms will 

 rise to the level of those in the surrounding areas ; at first 

 this will not have the effect of lifting the crust, for its 

 strength has been greatly increased during subsidence — 

 partly, its curves are directed towards the pressure from 

 below ; partly, its thickness is increased by the addition of 

 seven to nine miles of sediment. The resistance thus offered 

 affords time for the temperature to rise through the mass 

 until some of the upper strata become sufficiently heated 

 to rise in anticlinal ridges, to be followed by those below, 

 until the archsean crust itself is reached, when this also will 



