academy of sciences] SCIENTIFIC RELATIONS 139 



This conclusion was enforced by the statement of an elementary physiographic principle, 

 which Gilbert seemed to feel had been insufficiently considered by paleontologists: 



When a surface shaped by some other agent than the atmosphere ... is exposed to atmospheric agencies, 

 its sculpture begins. For a long time its original features continue to be the characteristic ones, but they eventu- 

 ally become subordinate and finally disappear. The original forms are at first new and fresh, then old, worn, 

 and hard to discover; and finally the fact that they once existed can be known only from the internal structure 

 of the deposits to which they belonged. 



It is striking testimony to the recency with which the rational study of land forms had then 

 been entered upon to find that principles so simple as these were deemed important enough by 

 so profound a student of geological philosophy as Gilbert, as to warrant their presentation at a 

 National Academy meeting! The annual report of the academy notes that Gilbert's paper was 

 discussed by Cope, Marsh, Powell, and Gill; but no record is preserved of their scintillating 

 remarks, nor of the measure of acceptance given by the paleontological three of the four to 

 Gilbert's physiographic lesson. 



JOINTS IN BONNEVILLE CLAYS 



During the first 10 years of Gilbert's residence in Washington as a member of the national 

 survey, he was so occupied with the duties of his office that he seldom had leisure to write out 

 in full the substance of the communications made at meetings of scientific societies; even the 

 results of his field work in western New York were inadequately published, as will appear in the 

 sections devoted to the problems there investigated; it is therefore not surprising that he seldom 

 found time for the preparation of independent articles for scientific journals. One of the few 

 subjects that he treated in this way was the origin of joints, with particular relation to those 

 that he had seen in the Bonneville clays. 6 His discussion of this topic is objectively a useful 

 contribution to a difficult problem, but it is here of greater interest as an illustration of his capac- 

 ity and his habit, one might almost say, his preference for maintaining a suspended judgment 

 in matters regarding which any shade of doubt remained. He described the facts and then 

 examined the explanations that had been proposed for them: 



If the considerations here adduced have weight, then neither hypothesis [shrinking or compression] is satis- 

 factory, and the problem is an open one. It is certainly hard to correlate the parallelipipedons into which the 

 clays of the Salt Lake desert are divided with the polygonal prisms normally arising from shrinkage; and it is 

 especially hard to admit that the clays have been subjected since their deposition to coercive pressures from two 

 independent directions. In my judgment it is proper to conclude, first, that the joints are not due to shrinkage, 

 and second, that the theory which regards them as identical with slat}' cleavage and ascribes both to compression 

 is untenable. If pressure and compression suffice for the explanation of slaty cleavage, then jointed structure is 

 something distinct from cleavage and needs an independent explanation. If joints and cleavage are merely 

 diverse expressions of the same general structure, then the theory of slaty cleavage which has been so widely 

 received fails to comprehend all the facts and needs to be revised. 



It may be added that while this problem was in Gilbert's mind, his few diagrams of basin- 

 range structures represented their inferred marginal faults by vertical fines, this being the graphic 

 expression of his belief that their uplift resulted from vertical displacement without lateral 

 compression; and that it was from this indirect argument he excluded compression from all share 

 in the production of joints in the Bonneville clays. It was not until a number of years later 

 that he came upon certain features of the range fronts which indicate a slanting attitude of the 

 master fault planes, and it would seem that the oblique displacement of the mountain blocks 

 on such faults might well occasion a considerable amount of compression in the surface parts of 

 the blocks at least; but it is hardly possible that the small amount of oblique displacement in 

 post-Bonneville time could compress the clays sufficiently to joint them. 



HOME AFFAIRS 



It is sad to have to relate that during this period of growing scientific relations Gilbert's 

 home life was clouded by a heavy affliction. For two years after returning from Salt Lake City 

 his family had no fixed residence in Washington, and it was during this unsettled period that 



• Postglacial joints. Amer. Journ. Sci., ixiii, 1882, 25-27. On the origin of jointed structure. Ibid., uiv, 1882, 50-53; xivii, 1884, 47-49. 

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