78 GROVE KARL GILBERT— DAVIS |MEM0IBS [vo L TI ^t 



Rapidly as the report was prepared, it still ranks to-day as a masterpiece of logical and 

 geological analysis. Geologists of the younger generation who have encountered laccoliths 

 chiefly on the pages of a textbook along with faults and unconformities, as parts of the stand- 

 ard material of their science, will do well to make acquaintance with the original monograph 

 in which the existence of these peculiar igneous structures was first demonstrated a little over 

 40 years ago. Geologists of an older generation will also profit by turning again to the report 

 which they must have first read with immature eyes, for a rereading of it will probably discover 

 many ideas which their memory has not retained, and possibly not a few lessons which they 

 did not clearly apprehend before. But quite apart from the content of the report, its method 

 and its manner deserve attentive examination by anyone who is searching for a good example 

 of scientific investigation, for it is a model of penetrating interpretation and candid exposition; 

 and it may be particularly recommended to students of philosophy who, already familiar with 

 the principles of logic in the abstract, wish to study a worthy example of then* application to a 

 concrete scientific problem. If such students are partisans of the absolute school, they will of 

 course condemn Gilbert's adoption of one set of speculations as the basis for another set, and 

 they may even be amused by the credulity of geologists in general who so willingly accept even 

 the later set of speculations as "conclusions''; but if they are of the pragmatic school, they 

 will rejoice over the ingenuity with which Gilbert succeeded in revealing so many conditions 

 and processes of the past on the basis of so short a study of the present. 



OBSERVED AND INFERRED STRUCTURES 



Gilbert learned the general structure of the Henry Mountains by "a few comprehensive 

 views from the mountain tops," the highest of which rise some 5,000 feet above the surrounding 

 surface, thus attaining altitudes of over 11,000 feet. This method of geological investigation 

 may seem hasty if not superficial to an observer bred in a plant-covered region, but it will be 

 accepted as satisfactory and convincing by an observer familiar with the frank confession 

 made by the rocks of "a naked desert, soilless and almost plantless," as to their succession and 

 attitude. The mountain group as a whole occupies a space measuring about 30 miles north- 

 south by 10 miles east-west; and each of its 36 members represent a laccolith 1 or cisternlike 

 mass of igneous rock, still more or less covered by the lower beds of the heavy series of strata 

 beneath which it was originally intruded. The stratified rocks of the district are nearly hori- 

 zontal, except where blistered up by the intrusions. Their total thickness from the upper 

 Carboniferous into the mid- Cretaceous is about 7,000 feet; several thousand feet of Tertiary 

 strata, still seen in the high plateaus on the west, are here wanting by reason of widespread 

 erosion. The laccoliths consist of "trachyte" according to the terminology adopted by Gilbert; 

 a rock that might now be called a " noncrystalline quartz monzonite porphyry." They occur 

 chiefly at two levels or zones in a lower and a higher series of shales, each series between being 

 1,000 feet or more in thickness, and the two being some 3,000 feet apart vertically. The mean 

 diameter of 8 well-defined laccoliths in the lower zone is 2.6 miles; and of 10 in the upper zone, 

 1.2 miles; their thickness is usually from a sixth to a quarter of their diameter; but one of 

 them, Howell, has a diameter of 2,000 feet and a thickness of only 50 feet. The volume of the 

 largest, Hillers, is estimated at 10 cubic miles. The arrangement of the different laccoliths is 

 without discernible system, and they would "prove intractable in the hands of those geologists 

 who draw parallel lines through groups of volcanic vents by way of showing their trend." 



At about the time of Gilbert's first work on these singular mountains, certain geologists, 

 members of the Hayden survey, were studying other examples of a somewhat similar structure 

 in the Rocky Mountains of Colorado; but they will not be especially referred to here, as the 

 object of this memoir is biographical rather than geological. Even in the Henry Mountains, 

 the outward or quaquaversal dips of the strata by which the laccoliths are now more or less 

 enveloped had been recognized by two earlier explorers of that secluded region; one was 

 Steward, a geologist of Powell's boat party down the Colorado in 1871; the other was Thomp- 

 son, Powell's topographer, in 1872. But Gilbert was the first to detect the existence of hori- 



1 This form of the word, suggested by Dana, has generally replaced Gilbert's original spelling, laccolite, and is here adopted eieept in quotations 



