622 



REPOKT OF NATIONAL MUSEUM, 1904. 



region he believed to be confirmatory of Richthofen's law of the 

 sequence of volcanic rocks.® He noted his objections to the German 

 method of rock classification based upon geological age. 



In 1876 G. K. Gilbert, then a member of Major Powell's corps, made 

 a study of the Henry Mountains of Utah. His report appeared in 



1877 in the form of a quarto 



volume of 160 pages, with five 



folding plates, and 



Gilbert's Work in the ° r ' 



Henry Mountains, numerous plates 

 1875-1877. \ 



and figures in the 

 text. In this he showed that these 

 mountains were due to the intru- 

 sion from below of igneous mat- 

 ter through Carboniferous and 

 Triassic strata, causing the over- 

 lying Cretaceous and Tertiary 

 beds to arch or bulge upward, 

 the present aspect of the moun- 

 tains having resulted from erosion, 

 whereb} 7 the overlying bulged 

 beds and a portion of the igneous 

 rocks as well were cut away. 

 Phenomena of this type had been 

 previously noted by Holmes, 

 Peale, and Marvine, of the Hay- 

 den Survey (pp. 601, 602), but it re- 

 mained for Gilbert to fully elabor- 

 ate the idea, and that in a man- 

 ner that must ever connect it with 

 his name. To intrusions of this 

 type he gave the name of lacco- 

 lites. 



In this report Gilbert showed 

 that, in the uplifting of the sand- 

 stone to form these domes, the 

 beds, as in the case of that of the 

 Vermilion Cliff sandstone, might 

 be elongated as much as 300 feet 

 in a distance of three miles. That this could be done, and that sud- 

 denly, he believed to be due to the pressure of the overlying beds. 

 The material was in a quasi plastic condition, and no fissures could 

 be opened unless coincidently filled by some material which would 



«Richthofen divided the Tertiary and post-Tertiary volcanic rocks into the five 

 classes given above, and regarded them as having been the product of a regular 

 sequence of eruptions, propylite being the most ancient and basalt the most recent. 



Fig. 119.— Laccolites. (After G. K. Gilbert. 



