MOVEMENTS OF KOCKS, ETC. 791 



movements, and that this takes place without any appreciable 

 metamorphosism ; but it is difficult to find opportunity for the 

 demonstration of the phenomenon by measurement. . . . Of the 

 unfractured quaquaversals of the Henry Mountains there is one 

 which combines all the essentials of a crucial case. The ' Lesser 

 Holmes ' arch is nearly isolated ; on three sides it rises from the 

 undisturbed plateau, and on the fourth it joins a similar but 

 fractured dome. The major part of its surface is composed of one 

 bed, the Yermilion Cliff Sandstone, broken only by erosion. 

 Comparing the length of this bed in its present curved form with 

 the space it must have occupied before it was upbent, I find that in 

 a distance of three miles it has been elongated 300 feet "*. 



Second, as to the diffusion of lava between some of the beds which 

 have been penetrated. 



This may evidently be attributed to the lava having a less specific 

 gravity than that of the strata which it penetrated, and to a lack of 

 cohesion between some of the invaded layers which allowed portions 

 of strata that were weakened by vertical fissuring to break away 

 from the better-supported rock above, and gradually to bend down 

 while the liquid lava passed into the horizontal rift which was thus 

 forming. 



Third, as to the elevation of the upper crust to form protuberances 

 on the surface. 



If the fissures had extended through the crust, the lava would 

 have passed up, and, having a specific gravity less than that of the 

 crust, would have welled out over the surface. 



Now, suppose a very thick layer of plastic extensible clay had lain 

 ,at the surface, and that the fissures formed extended through all the 

 strata except the clay. The lava would in this case also have 

 welled up, though while the clay remained unbroken it would not 

 reach the surface, but would push up the clay ; and it would 

 continue to act thus until the hydrostatic pressure downwards, 

 through the fissure, of the accumulating lava and the clay resting 

 upon it balanced the hydrostatic pressure upwards caused by the slow 

 sinking of the fissured crust. 



The same line of argument manifestly applies to any case in 

 which, as in that undei? notice, the upper crust is sufiiciently 

 flexible to yield to the pressure brought to bear upon it. 



The reason why, in the Henry Mountains, the crust yielded in 

 such a way as to produce the wonderful effects recorded is because in 

 that particular spot the lava became very extensively diffused in 

 wide sheets between the layers of strata. JFor we see that an 

 exceptionally great diffusion of lava in this way must inevitably 

 expose large surfaces to upward pressuref, at places where the solid 

 crust has less thickness and therefore less resisting power than in 

 the region around. 



I will now call attention to some phenomena seen in these 



* ' GreologY of the Henry Mountains,' p. 80. 



t As Mr. Grilbert remarks, the action of the liquid lava was exactly that of 

 the water in a hydrostatic press. See ' Geology of the Henry Mountains,' p. 95. 



