462 



NA TURE 



[March 19, 1896 



boy, in demonstrating geometrical propositions, was not the 

 least put about with the reversal of the diagram. He afterwards 

 turned out a clever pattern-designer. James Shaw. 



The Age of the Wealden. 



In a letter in Nature for March 12, Prof. Marsh calls 

 attention to the evidence of the vertebrate fauna of the 

 Wealden rocks as being in favour of including that series in the 

 Jurassic system. 



It may be of interest to add that a recent examination of an 

 exceedingly rich collection of fossil plants obtained by Mr. 

 Rufford from the Wealden rocks near Hastings, and now in the 

 British Museum, leads very decidedly to the same conclusion. 

 Between the Wealden flora of the South of England, in which 

 no traces of undoubted Angiosperms have so far been found, 

 land the typical Jurassic plants from the Yorkshire coast, there 

 is a very close resemblance. A. C. Seward. 



Cambridge, March 16. 



The Stress in Magnetised Iron. 



Dr. Chree will meet, I think, with general support in his 

 opposition to the view that there is of necessity, or even 

 usually, an actual stress in a magnetised rod tending to shorten 

 it ; but in maintaining, as I understand him to do, the opposite 

 view that the magnetic tension along the lines of force is 

 necessarily accompanied by a mechanical stress of pull and the 

 associated extension, he appears to me to be on more disputable 

 •ground. Dr. Chree's conception of the Maxwell distribution of 

 stress seems, if I may venture to say so, to be too materialistic. 

 What Maxwell really showed, of course, was that such a 

 ■distribution would produce on every element of matter in the 

 field the mechanical force which it was known actually to 

 experience. It was not suggested, however, that these stresses 

 were to be considered as transmitted by the matter by virtue of 

 its mechanical properties, indeed this could clearly not be the 

 case where the matter was liquid or gaseous ; and so there are 

 no grounds for supposing that the matter would exhibit strains 

 -directly associated with these stresses. The stresses, in fact, 

 must be considered as transmitted by the ether which pervades 

 the field, and it is in the ether that the associated strains are to 

 be looked for. 



If, however, we turn our attention from the ether to the 

 matter in the field, and remember that certain portions of this 

 matter will in general be experiencing mechanical forces, we 

 see that if its equilibrium is to be maintained a suitable system 

 •of mechanical stresses and the associated strains must be set up 

 in it. 



In the case considered by Dr. Chree in his letter to Nature, 

 published on January 23, it is plain that if A A' and BB' are air- 

 gaps, and are not filled up with a material capable of offering 

 resistance to longitudinal compression, the portions A' and B' of 

 the bar must be held or fixed in some way if there is to be 

 equilibrium. This was pointed out by Prof. Ewing, but Dr. 

 Chree does not seem to have appreciated its significance, and 

 his disregard of the external forces required for this purpose is re- 

 sponsible for the apparent discontinuity to which he refers in his 

 second letter. If A' and B', with these forces applied to them, are 

 allowed to move up to A and B so as to close up the air-gaps, 

 we pass without discontinuity to the case of a magnetised rod 

 •under external pull, and thus in a state of mechanical tensile 

 stress and elongation. If now we consider these external forces 

 to be gradually diminished to zero, and suppose that the 

 •question is not complicated by end effects at the outer ends of 

 A' and B' (as it will be in the case of a straight rod unless 

 external forces are kept applied at these ends), the mechanical 

 tensile stress and elongation will diminish to zero also, and we 

 have passed without discontinuity from Dr. Chree's result to 

 Prof. Ewing's. 



That Dr. Chree has obtained the correct result for the special 

 case which he investigates, appears from the following considera- 

 tions. Assuming that AA' and BB' are equal air-gaps, and that 

 A' and B' are fixed, AB will be in equilibrium. The Maxwell 

 distribution of stress gives equal mechanical forces on the 

 surfaces A and B directed outwards, and consequently there will 

 be tensile mechanical stress in AB with its associated extension. 

 In this case, therefore, there is actual elongation of the metal 

 in the direction of the lines of force. In the case of an endless 

 ring, however, the Maxwell distribution of stress gives no 

 mechanical force, and no mechanical stresses with associated 



NO. 1377, VOL. 53] 



strains will be set up. This is the case considered by Prof. 

 Ewing, who obtained the same result. I must confess that I do 

 not follow Dr. Chree's objections to Prof. Ewing's reasoning on 

 this point. The mechanical stresses must be such that every 

 portion of the ring is in equilibrium. Prof. Ewing does not 

 complain that tensile stress in a ring is unimaginable, but that 

 it does not comply with this condition. If Dr. Chree will re- 

 consider his reference to the case of a rotating anchor ring, he 

 will admit, I think, that as every element of such a b6dy is not 

 in equilibrium, but in accelerated motion, the fact that tensile 

 stress can and does exist in it is not relevant. 



As a further illustration of the variety of the mechanical 

 actions which may accompany the Maxwell distribution of stress, 

 we may consider the case of two sheets of tinfoil placed against 

 opposite faces of a plate of glass and maintained at different 

 potentials. It is readily seen that in this case the glass under- 

 goes a mechanical stress of compression and the associated 

 strain of shortening in the direction of the lines of electro- 

 static force, though the Maxwell stress in this direction is a 

 tension. L. R. Wilberforce. 



Cavendish Laboratory, Cambridge, February 26. 



RECENT WORK OF THE GEOLOGICAL 

 SURVEY OF THE UNITED STATES} 

 U. 

 C INGE Gilbert (in 1887) published his classical mono- 

 •^ graph on the geology of the Henry Mountains, in 

 which he gave to the world for the first time a clear and 

 connected account of the nature and occurrence of 

 Laccolites, we have waited many years for further 

 original work on this type of physical structure in 

 America. A study of the writings of Peale, and of 

 the exquisite panoramic drawings of Holmes, made 

 it abundantly clear that laccolitic masses must be fre- 

 quent in the United States. Mr. Whitman Gross has 

 now collected a number of instances from Colorado, 

 Arizona, and Utah.^ He has remarked on their structure 

 so far as it has been made out by these observers, by 

 Emmons, and by himself, given a description of the 

 characters of the rock of which the laccolites consist, 

 and offered some remarks on the general theory of lac- 

 colitic structure. 



The theory has not been quite so fertile in results as 

 might have been expected from the clear-cut nature of 

 Gilbert's brilliant piece of work ; nevertheless the author 

 is able to show that, although Reyer refuses to accept 

 the facts on which the theory is based, Suess, on the 

 other hand, reproduced the illustrations, summarised the 

 results of Gilbert, Peale, and Holmes, and applied them 

 to European and other examples, while Neumayr added 

 further arguments in favour of the intrusive nature of the 

 Henry Mountain rocks themselves. It may be pointed 

 out here that the existence of laccolites has been taken 

 almost for granted by many British authors (not referred 

 to by Gross), such as Kinahan, Geikie, Harker, Marr, 

 and others, and that in one case at least a numerous 

 group of laccolites has been described, and proof given 

 that the igneous masses are conformably underlain and 

 overlain by sediments.^ The abstract of this paper, all 

 that was published for many years, also anticipates some 

 of the conclusions independently reached by Mr. Cross. 



The familiar Henry Mountains are first described, then 

 the West Elk Mountains to which attention was drawn 

 by Peale, and in succession the San Miguel, La Plata, 

 Garriso, El Late, Abago, and La Sal Mountains. All 

 these are either on the verge of the great plateau, being 

 thus geographically outliers of the Colorado Mountains, 

 or they are isolated groups on the plateau itself They 

 are groups of laccolites intruded into nearly horizontal 

 strata probably at about the same period — in Tertiary 



1 Continued from page 441. r - i 



- " Fourteenth Annual Report of the Geological Survey of the Umted 

 States," 18Q2-93. (1894.) 

 3 " Report of the British Association," 1886. 



