academy op sciences] WHEELER SURVEY 31 



Then after a page of other matters : " P. S. I find that it is Powell who has an opinion about 

 the Adirondacks"; as if in the multiplicity of new ideas this one had been forgotten. A later 

 instance of a somewhat similar kind will be told in connection with the problem of laccolites. 



A digression may be made here to introduce a striking statement concerning the great break 

 between the Archean rocks and the later formations, a statement that is noteworthy for the 

 importance of its content as well as for the strength of its induction. It is taken from a review 

 of Geikie's Textbook of Geology, which Gilbert wrote in 1885: 



The unconformability between the Archaean and the Palaeozoic [it may be inferred that this classic diph- 

 thong was introduced by the editor of Nature] is not mentioned in such a way as to convey an impression of 

 the profoundness of the chronological break. There is no known locality where a newer formation rests con- 

 formably upon the Archaean. There are few where the discordance of dip is not great. There are few where 

 the superior formation is not relatively unaltered, and none where the inferior formation is not highly meta- 

 morphosed. So far as we know, the Archaean strata were both thrown in great folds and plicated in detail, 

 were universally subjected to a metamorphism such as in later rocks seems to have been accomplished only 

 at a depth beneath the surface, and were subsequently worn away upon a most stupendous scale before they 

 received any sedimentary covering within the regions now accessible for examination. Compared with this all 

 other chronological breaks are trivial, and we may almost say that, compared with this, all other stratigraphical 

 breaks are local. 3 



It would appear from such a passage as this that, although Gilbert was as a rule little 

 occupied with historical geology, he had a fine appreciation of its greater lessons. 



VOLCANIC ROCKS AND STRUCTURES 



Volcanic forms will be referred to on a later page; volcanic rocks and structures may here 

 be passed over briefly, for although they were duly attended to wherever encountered, they were 

 not in after years subjects of Gilbert's special studies. He rarely concerned himself later with 

 the distinction of trachyte, rhyolite, and basalt; and indeed the mention of these and other 

 kinds of lavas in his early reports is chiefly noteworthy in connection with his endeavor to test, 

 by examination in the field, Richthofen's then recently announced "natural order of sequence" 

 for eruptive rocks which "had been before considered almost exclusively from a chemical and 

 lithological point of view" (131); noteworthy also because, while the repeated references to 

 Richthofen would appear to constitute an exception to the rule that Gilbert did not cite the 

 work of foreign authors, it is not so in reality; for the work here referred to was based chiefly 

 on observations made by the distinguished German geologist and geographer when he was 

 crossing the western United States on his way to China in the sixties, and his " Natural system of 

 volcanic rocks" was first published in the memoirs of the California Academy of Sciences in 1868. 



As to Gilbert's field test of the sequence of volcanic rocks in the Great Basin, basalt was 

 always found to overlie trachyte and rhyolite, but the succession of these two seemed variable; 

 the earlier members of the series, propylite and andesite, were rarely met (131, 132). In the 

 plateau region "the invariable order of superposition is: Basalt, sanidin-dolerite, trachyte." 

 A sententious statement of ponderosity hardly paralleled in later writings is made concerning 

 the second member of this series: 



The name "sanidin-dolerite" is used, for merely temporary convenience, to designate a rock of considerable 

 importance in Arizona, which seems to fall without our present nomenclature and deserves the careful scrutiny 

 of the lithologist. ... It is quite possible that when, by the determination of the constitution of its matrix, 

 it is fully denned, it will not appear lithologically entitled to a specific appellation, but the recognition of its 

 individuality finds geological warrant in Arizona (526). 



Concerning the relative abundance of different volcanic rocks, it is said that on the plateaus 

 basalt covers large areas in relatively thin sheets and there rivals trachyte in abundance; but 

 in the Great Basin basalt, although assuming an apparent importance on the map from being 

 of latest date, shrinks into insignificance when its volume is compared with the massive erup- 

 tions of trachyte and rhyolite; these rocks, rising from few issues, have formed huge bosses, 

 often of great thickness, divided by few or no bedding surfaces. Even though now much reduced 

 by erosion, they still remain in immense masses (127, 128). The use of the German word, 

 "Trass," for a volcanic ash deposit (540) was probably an echo of Cosmos Hall. 



* Nature, xrrii, 1885, 261. 



