ACADEMY OF SCIENCES] POWELL'S SURVEY 89 



The result gained varies from 10,300 to 14,500 feet, according to the relation that is assumed 

 to exist between the thickness of the cover and its resistance to flexure; and as the present 

 cover of the upper zone averages only 3,500 feet in the thickness, the original cover, presumably 

 Tertiary for the most part, must have lost from 7,000 to 11,000 feet of its thickness by post- 

 intrusional erosion. As the lost strata are still present in the high plateaus to the west, and as 

 independent geological evidence suggests their original extension over the Henry Mountains 

 area, the calculated thickness of the original laccolith cover seems reasonable. 



Yet in spite of the plausibility of his argumentation, Gilbert did not insist upon the accept- 

 ance of the results that it reached. He wrote: 



I am far from attaching great weight to this speculation in regard to the original depths of the laccolite 

 covers. It is always hazardous to attempt the quantitative discussion of geological problems, for the reason 

 that the conditions are apt to be both complex and imperfectly known; and in this case an uncertainty attaches 

 to the law of relation, as well as to the quantities to which it is applied. Nevertheless after making every allow- 

 ance there remains a presumption that the cover of the laccolites included some thousands of feet of Tertiary 

 sediments (94). 



This is putting it mildly, to say the least. It would not be overstating the case to say that 

 a Henry Mountains laccolith is not, as usually understood, simply an irresponsible mass of 

 intrusive rock, but in view of Gilbert's well-conducted analysis it is a comparatively orderly and 

 reasonable structure, the general position and dimensions of which appear to be explicable on 

 mechanical principles. 



GENETIC DEFINITION OF A LACCOLITH 



The largely inductive or empirical definition of a laccolith given at the end of an earlier 

 section may now be replaced by an expanded genetic definition : A Henry Mountains laccolith 

 is a mass of intrusive igneous rock which, supplied through an inferred chimney from a source 

 of unknown depth, begins its expansion by spreading laterally in a horizontal fissure-plane, 

 apparently in accordance with the hydrostatic law, at a level where, while still molten, its den- 

 sity is less than the average density of the underlying and greater than that of the overlying 

 strata. In spreading, the molten magma first assumes the form of a roughly circular sheet or 

 disk of small thickness, but as the radius of the disk increases it attains a dimension at which 

 the intrusive force of the magma is more economically applied to the thickening of its mass in 

 domelike form by abruptly flexing up the strata around its margin and lifting up the strata 

 above it, than to spreading farther and lifting up a larger total area of overlying strata with- 

 out so much flexing. As marginal flexing takes place, the flexed strata are stretched by being 

 squeezed between the upward pressure of the intruding magma and the downward weight of the 

 higher strata. 



The limiting diameter at which the abrupt up-flexing of the marginal strata takes place 

 increases with the depth at which the magma outspreads; the lower-lying laccoliths of the 

 Henry Mountains have an average diameter of 2.6 miles under an estimated original cover 

 nearly 3 miles thick; and the same figures for the higher-lying laccoliths are 1.2 and somewhat 

 more than 2 miles. If the upward pressure of the magma is maintained, the laccolithic dome 

 will gain in thickness by continuous or by successive intrusions until the total weight of the 

 laccolith and its cover is greater than the upward pressure of the magma; when this stage is 

 reached, the magma will seek another place of ascent and intrusion. The thickened form of the 

 completed laccolith is not an indication that the magma was imperfectly fused at the time of its 

 intrusion, but that it was constantly "solving the problem of 'least work' "; for its effective 

 fusion is proved by the thinness of certain sheetlike laccoliths which did not thicken, as well as 

 by the narrowness of the many dikes which appear to have been instantly shot into any fissures 

 opened in the overlying beds. The formation of laccoliths, as thus explained, leads to the 

 conclusion that they are not associated with volcanoes, beneath which, according to the hydro- 

 static law, the rising magma should be in its molten state less dense than the rocks through 

 which it rises, whatever its density may become after its gases have escaped and it has been 

 cooled and solidified. 



