90 GROVE KARL GILBERT— DAVIS [toM ™ s rv<^xxt 



gilbert's theory not generally understood 



If the second 50 pages of the Henry Mountains report mean anything, they mean that 

 Gilbert attached about as much importance to the highly theoretical inferences which they 

 contain concerning the conditions and processes of laccolithic intrusion as he did to the simpler 

 inductions presented in the first 50 pages concerning the actual structure of laccoliths. It 

 therefore occasions some surprise to discover that many geologists, while accepting his induc- 

 tions as to the structure of laccoliths and his inferences as to their subterranean intrusion, have 

 given little heed to his speculations as to the processes of their origin, apart from the elementary 

 item that intrusion actually took place. A further statement of this aspect of the problem is 

 here undertaken, not so much with the purpose of advancing the understanding of a special 

 geological problem, as with the desire to promote a fuller appreciation of Gilbert's thought 

 upon it. 



We are not here concerned with the doubts frankly expressed by Green and Neumayr as 

 to the verity of laccolithic intrusion, or with Reyer's erroneous preconception that the igneous 

 masses of the Henry Mountains are extrusions; sufficient references to these authors have 

 already been made. We are still less concerned with the unappreciative comment made by 

 De Lapparent, who after spending several pages in combating the obsolete theory of craters of 

 elevation, briefly discredits Gilbert's novel and valuable interpretation of laccoliths, because it 

 involves intumescence, "pour F explication de laquelle on propose une hypothese depourvue 

 jusqu'ici de toute verification directe"; 2 or with the careless misrepresentation made by Haug, 

 who takes to himself the explanation by intrusion and intimates that Gilbert had advocated an 

 eruptive origin: "On a suppose tout d'abord que le magma fluide s'etait precipite dans des 

 creux preexistants, resultant du decollement des strates superieures, d'ou lenom de laccolithes 

 que leur a donne Gilbert. Mais U est bien plus probable que le magma s'etait introduit entre 

 deux couches en soulevant la couche superieure." 3 Misunderstandings of these kinds do not 

 need special comment, for they are not likely to endure; but attention must be given to certain 

 incomplete statements by other geologists who, wlule accepting the most manifest parts of 

 Gilbert's theory, fail to do justice to its more delicate and ingenious elements. 



It is possible that some or all of these finer elements are debatable, for Gilbert himself 

 recognized that his method of analysis included several assumptions and approximations; and 

 20 years later, when he described a small laccolith in the plains of eastern Colorado, he referred 

 to some of his earlier conclusions as only "tentative"; but they were nevertheless framed with 

 good judgment and they were well grounded. Perhaps some of them are erroneous, although 

 they have not yet been shown to be so; surely, until they are shown to be without value, they 

 ought to be announced as inherent parts of Gilbert's views. There is no novelty in this 

 principle of impartial geological exposition; our more important textbooks traditionally present 

 both sides of other debatable questions; for example, the peculiar views of Semper, Rein, and 

 Murray, although they were observers of no particular geological competence, are still fre- 

 quently cited in paragraphs on the geological aspects of the coral-reef problem in standard 

 textbooks. Surely Gilbert's views on the laccolithic problem merit at least as fair a considera- 

 tion, for he was an exceptionally competent geological observer and thinker. 



Yet an admirable textbook, which deserves high rank by very reason of giving its readers 

 the opinions of different observers on debated problems — for example, the opinions of the 

 three observers just named in connection with the coral-reef problem — does scanty justice to 

 Gilbert's theory of the origin of laccoliths, concerning which it is merely said: "Large bosses of 

 trachytic lava have risen from beneath, but instead of finding their way to the surface, have 

 spread out laterally and pushed up the overlying strata into a dome-shaped elevation." The 

 student here finds nothing about the hydrostatic law in connection with the level of lateral 

 intrusion, and nothing about the advantage that a large intruded sheet has over a small one 

 in the production of a dome; but he does find in the next sentence a direct contradiction of 

 Gilbert's views that laccoliths and volcanoes do not occur in close association, for the further 



» Traits de Geologie, 5th Ed. 1906, i, 409. 

 • TraitS de Geologic, 1907, i, 276. 



