ORIGIN AND DEVELOPMENT OF ORE DEPOSITS. Ill 



This local heating, however produced, (and several not 

 improbable modes might be suggested) would be sufficient, in 

 some cases at least, to melt off the lower portions of the solid 

 crust if it had been previously bent down into folds or " sags " 

 by large additions to its weight. What was before especially 

 thick would now be rendered especially thin. Thus would result 

 a local weakness sufficient to determine the locus of a system of 

 contortions resulting in more definite elevations, especially if 

 conjointly aided by gaseous accumulations. By the pressure of 

 the related subsiding portions of the crust upon the underlying 

 molten mass, the rising folds of the contortions would be filled 

 by injections of portions of this mass. If the pressure were 

 great enough to rupture the crust the fracture might be repaired 

 by the solidification of the injected matter. The frequent exist- 

 ence of ridges of plutonic rocks along lines of elevation indicates, 

 either that they were actually so extruded in a fused state 

 through the fractured crust, or that, having filled and occupied 

 rising folds and arches in the contorted rock-crust, they are now 

 exposed by denudation. 



To rightly apportion the work done by subsidence and 

 elevation respectively, in any particular case, must always be 

 difficult, if not impossible — especially as their effects are compli- 

 cated by variations in density, occasioned by local heating — by 

 the rock-material passing from the solid to the liquid state, and 

 vice versa — by condensation of rock-material under pressure — and 

 by changes of volume in passing from the amorphous to the 

 crystalline state. It is indeed doubtful whether there has ever 

 been any real elevation of any considerable part of the earth's 

 surface — it may well be that the apparently elevated areas are 

 not really more distant from the earth's centre than before, but 

 only seem to be so, owing to the actual subsidence of the 

 surrounding regions. Such questions, though of great theoretical 

 importance, have little or no practical bearing on our present 

 enquiry.* 



*This question of the causes and effects of elevation and subsidence, and of 

 contortion, has been discussed very ably by Prof. C. Lloyd Morgan, in the 

 Geological Magazine for May, 1888. After discussing the effects of lateral 

 pressure in forcing the " upper layers " of the earth's crust into long geoclines, 

 and the consequent effects of the altered pressure of the rocks on the magma 



