CHAPTER IX 

 THE CONDITIONS AND PROCESSES OF LACCOLITHIC INTRUSION 



THE HENRY MOUNTAINS AS TYPICAL LACCOLITHS 



It is as true to-day as it was when Gilbert wrote in 1877 that, while larger bodies of intru- 

 sive rocks occur elsewhere, no other examples so typical of simple laccolithic structure as the 

 Henry Mountains have been described. Had his study not been carried further than the 

 account of observed and inferred structures presented in the first 50 pages of the published 

 report, summarized above, it would have been a valuable contribution to American geology; 

 indeed to the geology of the world. But instead of stopping with existing structures, he under- 

 took in the second 50 pages a penetrating discussion of the conditions under which the lacco- 

 lithic intrusions of the Henry Mountains had taken place. His treatment of this problem is 

 delightfully ingenious and open; the reader feels as if the author were inviting his company on 

 a speculative excursion, in which all pertinent facts and suppositions are to be candidly ex- 

 amined. The method of presentation is the very reverse of that obscurantist style, sup- 

 posedly appropriate for popular scientific story-telling, in which the writer, as if assuming 

 that the public can not understand geological reasoning, announces conclusions without giving 

 grounds for them; or, still worse, puts the cart of conclusion before the horse of inference and 

 even presents the cart as the pulling force, witness such a statement as: 



Geologists have discovered that the climate of the earth was, in a past stage of its history, much warmer 

 and moister than now; therefore plants then grew luxuriantly and coal beds were formed of their remains. 



Gilbert's method was precisely the opposite of this, in that he always presented facts and 

 inferences in proper order, left no points in obscurity, never glossed over a difficulty or stretched 

 an argument, and never attempted to impose his views as if by authority. Indeed for purity 

 and candor of reasoning, the chapter on the conditions of laccolithic intrusion as exhibited in 

 the Henry Mountains has few parallels in geological literature. It is not to be expected that 

 the chapter was complete or that its conclusions were so final that they should apply to laccoliths 

 in regions of disordered structure; but the discussion unquestionably deserves high praise for 

 its ingenuity in utilizing all pertinent and available knowledge. It will be here presented in 

 outline. 



The discussion opens with a review of the results reached concerning the structural features 

 upon which the hypothesis that the Henry Mountain laccoliths are intrusions is based, and then 

 adopts the hypothesis because it ' ' accords with all the facts that have been observed and unites 

 them into a consistent whole" (54). Next, the intrusive rocks of the laccoliths are compared 

 with the effusive volcanic lavas of the plateau province; the first are without exception acidic, 

 while the others are basic, Dutton being here called in as a petrographic expert. It is not 

 possible to combine the two phenomena of intrusion and extrusion as the result of the rise of 

 one magma — Gilbert of course used the old term, lava — for "the acidic type if extruded at the 

 surface would be an ordinary [not a porphyritic] trachyte; the basic type if crystallized under 

 pressure would be classed with the greenstones. The basis for the generalization is exceedingly 

 broad. ... In the Uinkaret Mountains [on the plateau north of the western part of the Grand 

 canyon] Professor Powell has distinguished no less than one hundred and eighteen eruptive 

 cones [all of them resting on horizontal strata which give no indication of being arched by 

 underground laccoliths], and in the Henry Mountains I have enumerated thirty-six individual 

 laccolites. In one locality basic lava has one hundred and eighteen times risen to the surface by 

 channels more or less distinct, instead of opening chambers for itself below. In the other local- 

 ity porphyritic trachyte has thirty-six times built laccolites instead of rising to the surface" (71). 



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