790 ME. W. BAELOW ON" THE HOEIZOISTTAL 



The elevations of the earth's crust which form these mountains 

 are the outcome, on an exceptionally large scale, of the common 

 phenomenon of lava penetrating but part way to the surface in 

 dykes, and then diffusing itself between the beds and forming 

 subterranean lakes or deposits of lava. 



In this case very large deposits were formed, the intrusion of which 

 lifted great thicknesses of superincumbent strata, and produced 

 huge dome-shaped elevations of the otherwise nearly horizontal beds. 

 These very regular protuberances were afterwards carved by 

 denudation into rugged outlines of ridge and canon. 



The chambers occupied by the , intruded trachyte are in some 

 cases over three thousand feet high. They have in each case been 

 made along a shaly layer in the formation where the cohesion was 

 least*. They occur at different levels in the strata, and the lowest 

 in geological position is 4500 feet below the level of the highest. 



Large as is the scale on which the effects have been produced, it 

 does not appear necessary to attribute them to the action of any 

 other force than the force of gravitation acting in the manner I have 

 already described. 



Thus, first with regard to the frequent phenomenon of dykes 

 stopping short before they reach the surface, a phenomenon of which 

 we have here such important examples. 



In most cases where a body of molten rock spreads and produces 

 dykes the solid rock immediately over the liquid mass will experience 

 the lateral pull first, and thus the vertical fissures which receive the 

 molten rock luiTL hegin to O'pen from below. And in cases where the 

 upper strata are more plastic than the lower, or where they form an 

 elevation on the surface, and thus are less completely attached to the 

 rocks around them, it wiU often happen that, while the lower strata 

 of the solid crust are fissured, the upper strata will make sufficient 

 movement with respect to the lower to avoid rapture. 



It would seem that the whole district about the Henry Mountains 

 has experienced a force which ruptured the lower strata and 

 extended the upper strata without breaking them. Thus Mr. Gilbert 

 says respecting this district t : "It seems as though the crust of the 

 earth had been divided into great blocks, each many miles in extent, 

 which were moved from their original positions in various ways. 

 Some were carried up and others down, and the majority were left 

 higher at one margin than at the other. But although they moved 

 independently they were not cleft asunder, -the strata remained 

 continuous, and were flexed instead of faulted at the margins of the 

 blocks "J. 



And further on the same writer adds : " It has been the opinion, 

 not only of the writer, but of other students of the displacements of 

 the West, that the ordinary sedimentarj^ rocks, sandstone, limestone, 

 and shale are frequently elongated as well as compressed by orographic 



* ' Geology of the Henry Mountains,' b^ G. K. Gilbert, p. 58. 

 t Ibid. p. 11. 



I The mountains stand within the province of the great flexures, but are 

 independent of them. 



