110 ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. 



dence may produce local elevation, for " as soon as the resistance 

 offered by the descending mass becomes greater than that offered 

 by the stationary parts of the earth's surface, on either side of 

 the descending area, the enormous pressure will tend to relieve 

 itself by the rise of any part or belt of country where the amount 

 of resistence is least."* Thus, paradoxical as it may appear, 

 depression may actually be the immediate cause of regional 

 elevation. 



If the elevations were produced hy pressures from below, as, 

 for instance, from the expansion of a gas, we might expect the 

 production of a dome, or of a single ridge. But, it is well 

 known that " axes of elevation" are generally bounded on one 

 or both sides by crumpled and contorted rocks, forming parallel 

 systems of anticlines and synclines, and these are plainly produced 

 by lateral pressures. The pressures exerted by the subsidence 

 of large areas of the "crust" being constant, will, given sufficient 

 time, be transmitted to considerable distances laterally through 

 the somewhat plastic rock-material, until these once more are 

 supported below — hence the crumplings of strata which are often 

 observable for hundreds of miles on one side or other of a great 

 mountain range. The shell first formed, therefore, and to a 

 continually diminishing extent all later additions to it, would 

 be subject to contortion and ultimate fracture from the effects 

 of gravity. 



But it is conceivable and even probable that elevation may> 

 in some cases, result from local heating of the rocks not connected 

 with subsidence. Such a local heating of a part of the earth's 

 crust, 50 miles thick, from a mean temperature of 600° 0. up to a 

 mean temperature of 1000° C, would produce, by expansion of the 

 rock substance, an elevation of between 1000 and 1500 feet. 

 The cooling down of the same mass would lower the surface by 

 the same amount. t By such agency we might explain the 

 gradual rise of part of Scandinavia, or the subsidence of Green- 

 land. 



*IUd. 



•(-There are, of course, other causes known to Geologists, which are also 

 competent to produce local elevation and subsidence. 



