840 



DYNAMICAL GEOLOGY. 



2. The lateral pressure from the expansion of rocks by heat. 



3. The lateral pressure due to the action of gravity, pp. 666, 814. 



2. Through upward pressure. — 1. By means of steam or expanding vapors, 

 as in volcanic regions, producing oven-shaped prominences over lava currents, and 

 bulgings in areas of solid beds of rock, p. 747. 



2. By means of a forced rising of the melted rock of a conduit or fissure, acting on 

 adjoining or overlying beds, pp. 747, 824. 1 



1 In the case of the Henry Mountains, briefly referred to on page 824, the horizontal 

 strata of the region, for a thickness of at least 10,000 feet, according to Mr. G. K. Gil- 

 bert (who observes that in the era of the eruption, from 3,500 to 7,000 feet of Tertiary 

 probably covered the Cretaceous formation), were bulged upward, in consequence of the 

 intrusion below of melted porphyritic trachyte, into round-topped elevations (or groups 

 of such elevations), rising abruptly out of the plateau to a height, in some instances, of 

 5,000 feet. The dip about the sides is often 45° and in some cases even 80°. Around 

 and between them the region is a flat plain. If the facts are correctly interpreted by 

 the able geologist who reports them, these are examples of the flexing of strata into 

 mountain domes by an upward flowing lava-stream. 



Since the conclusions have great geological importance, some additional explanations 



Fig. 1160. 



Ideal Section of a Laccolith. From Gilbert. 



are here given from Mr. Gilbert's Report. 

 (The Report bears the date 1877, but was 

 not issued until October, 1879, after the 

 preceding pages of this volume were in 

 type.) Fig. 1160 is an ideal section of 

 the bulged strata with the underlying 

 laccolith. The laccolith, as is seen, rests 

 on horizontal strata; and this has been 

 observed to be their actual position in 

 several cases. The thickness, or height, 

 is sometimes over 3,000 feet, and the 

 breadth is, on an average, seven times 

 Traces of three or four lines of bedding 

 From the laccolith, rise 



the height, but in one case, only three times 

 sometimes exist, indicating an intermittent flow in the lava 

 dikes of trachyte — which, however, are generally much more numerous than might be 

 inferred from the figure, and often pass up between layers of the overlying beds. The 

 sandstone adjoining the trachytic mass, both above and below it and bordering the dikes, 

 is usually more or less altered by the heat to the thickness of a foot or more. Erosion 

 has reduced the original domes to deeply gorged mountain peaks and ridges, and in 

 some of them has exposed part of the interior laccolith, so as to show its original sur- 

 face, while in one the whole stands bare and waterworn. The chamber occupied by the 

 laccolith was in all cases made along a shaly layer in the formation where the cohesion 

 was least. They occur at different levels in the strata, and the one lowest in geological 

 position is 4,500 feet below the level of the highest; the former is between Carboniferous 

 beds, and the latter between Cretaceous. 



It follows from the conditions represented that the propelling action upon the lava, 

 in the deep regions whence it came, was so powerful that in spite of friction along the 

 passage and the density of the lavas (at least 2*33), they were thrown for an unknown 

 number of miles up to the laccolith level; and then had energy enough left to lift, in the 

 case of the laccolith lowest in geological level, a mass of beds 10,000 feet or more thick 

 and 2-25 in average specific gravity (equivalent in pressure to 675 atmospheres) to a 

 height of 5,000 feet. Some accession to the force, however, may have come from vapors 

 derived from subterranean moisture or waters encountered on the way up. As Mr. 

 Gilbert states, the intrusion of the lava laterally into a chamber widened the area of 

 pressure, and thus enabled it, on the principle of the hydraulic press, to accomplish the 

 lift. 



The author adds to Mr. Gilbert's explanations that, with so powerful a flow, the lavas 



