ROCKY MOUNTAIN STRUCTURE. 25 



accord, and whose discussion would not be appropriate in a memoir like 

 the present, which has to do with observed facts and with theories only 

 so far as they are necessary for a proper comprehension of these facts. It 

 is an observed fact that in the great mountain systems are found the most 

 intense expression of the compression of the crust, in plications and in great 

 faults. It is also an observed fact that along- these lines of elevation and 

 of consequent fracturing of the crust, have occurred the most extensive 

 extrusions and intrusions of molten or eruptive rock, whatever may have 

 been their source — whether from a fluid center or from a fluid enveloj^e 

 between a solid center and a solidified crust, or from subteri'anean lakes 

 of molten rock at different and varying points beneath the crust. It may 

 likewise be considered a fiict of observation that the tangential or horizontal 

 thrust which the contraction theory requires most readily accounts for 

 the plication and faulting of the sedimentary beds which geological study 

 discloses. This thrust may be best conceived as the expression of two 

 forces of compression : a major force acting at right angles to the longi- 

 tudinal axis of the mountain system, or east and west, and a minor force 

 acting in a direction parallel with that axis, or north and south. 



The geological structure of the Rock}' Mountains forms as marked a 

 contrast to that of the regions adjoining it on either side as do its topo- 

 graphical features. On the Great Plains, which stretch in an almost unbroken 

 slope from their eastern base to the Mississippi River, or, it might be said, 

 to the western foot of the Appalachians, the strata which form the surface lie 

 in broad undulations, whose angles of dip are so gentle as to be scarcely 

 perceptible to the eye, and which are apparently broken by no importfint 

 displacements. 



In the Colorado PhUeau region, whicli extends from their western edge 

 to the base of the parallel line of uplift of tlie Wasatch, the beds seem as 

 horizontal as when they were originally deposited, but along certain lines 

 abrupt changes of level are Jorought about by sharji monoclinal folds, accom- 

 panied by or passing into faults, and having great longitudinal extent. 



In the intervening mountain region the strata are compressed against 

 the original huid masses and flexed until the limit of tension is reached, 

 when by great displacements, often measured by thousands of feet, tlieir 



