IGNEOUS UPLIFTS. 209 



they were formed before or diiring or after the uprising of tlie great arcli, 

 there is notliing in their distribntion to suggest that they are in an)- wise 

 dependent upon it or closely related. Some of them are near the crest of 

 the arcli, some are where the rocks are stee^dy inclined at the side of the 

 arch, many are on the gentle slopes of the northern prolongation, and at 

 least one is entirely outside of the Hills. They are so nuich smaller tlian 

 the protuberance on which they rest that they seem to be merely superiicial 

 j)henomena — a sort of skin disease upon the surface of a tumor. 



But however insignificant the volcanic disturbances may appear when 

 contrasted with their vast orographic associate, they are by no means 

 unimportant Avlion considered by themselves. About each of the igneous 

 cores the upbent strata expose circular outcrops. Evidently their present 

 condition has resulted from protracted waste under the play of the elements, 

 and if we could restore the portions which have disappeared to the form 

 which they had immediately after the upthrust of the molten rock, we 

 should have mountains of very respectable proportions. It is safe to say 

 that when the trachyte of Warren Peaks Avas forced up beneath the Pots- 

 dam sandstone, it lifted it to a height of 3,500 feet, carrying with it all of 

 the strata that then rested upon it, and forming a mountain 3,500 feet in 

 height and six or eight miles in its smallest diameter. 



In the accompanying diagram, Figure 26, all uplifts known to be 

 caused by volcanic action are ignored. The orographic uplift, pure and 

 simple, is represented by the method of contours. It has no pretension to 

 minute accuracy, but it may be trusted to give a veracious rendering of 

 the main features of the displacement. The outermost curve of all is 

 intended to be drawn at the limit of the displacement, and represents the 

 base of our ideal dome where it joins the plane of reference. The next 

 curve is the 500-foot contour, and represents the intersection of our ideal 

 dome with a plane parallel to the datum plane and 500 feet above it. So 

 of all the other ciirves, the vertical interval between two adjacent lines 

 being in each case 500 feet. The courses of the two forks of the Cheyenne 

 are marked upon the diagram, partly to aid in giving it a geographical 

 position and partly to illustrate the fact, to which we shall again have 



Ubh 



