by Sedimentary Loading and Recurrent Expansion. 489 



in which these materials for inountain-buiiding were accumu- 

 lated, were in some cases, on the final completion of sedi- 

 mentation, double the depth of the deepest known oceanic 

 troughs, which do not reach more than 5 miles. 



Considering that there is a strong development of Cre- 

 taceous and Tertiary rocks extending along the western coast 

 of North and South America, it is seen that these operations 

 have there been carried on on an unusual scale. Deposit and 

 alteration of level, elevation and subsidence, but preponder- 

 antly subsidence, progressed for an immense length of geo- 

 logical time in these areas, occupying not a mean portion of 

 the Earth's history. 



It is not, however, to be assumed that this was a continuous 

 trough at any one time, rather that it consisted of a series of 

 connected basins which underwent independent changes of 

 level, the area being part of the time low-lying land inter- 

 changing with conditions of submergence. 



Volcanic Action often Contemporaneous ivith the Laying-down 

 of Materials for Mountain-Building. 



Contemporaneous intrusive sheets of volcanic rock are a 

 common occurrence in some part of the sedimentary history 

 of a mountain-range. In addition, it is frequently found that 

 volcanic ashes laid down in water, or subaerially, have a 

 large development in rocks composing mountain-ranges ; and 

 necessarily, if these occur, dykes and volcanic rocks of the 

 same age must exist in the foundation materials of the range. 



Dynamical Principles. 

 Every theory which has hitherto been proposed to account 

 for the elevation of mountains and the folding of the stratified 

 beds forming the Earth's crust hinges finally on changes of tem- 

 perature. Thus the tangential force generated in a rigid crust 

 of low temperature by the cooling and shrinking of the Earth's 

 nucleus has been invoked to account for the crumpling of the 

 crust into mountain-ranges ; the crumpled skin of a dried 

 apple being the stock illustration. In this case, the force 

 called in is continuous contraction by loss of heat. The theory 

 which 1 have elaborated is one dependent upon alternations 

 of temperature in the crust, contraction and expansion both 

 being agents of uplift and lateral pressure. 



Basins of Deposition and Loading of the Earth's Crust. 



It has already been shown that the establishment of basins 

 of deposition is the condition precedent to the building of a 



