488 



NATURE 



[Sept. 2o, 1883 



the red spot formerly was is now very white ; it passed over the 

 central meridian of the planet this morning at 4I1. 36m. (M.T. 

 at Palermo), which give*; for this place the Jovicentric longitude 

 63 , plainly corresponding to the longitude that Mr. Marth 

 assigned to the red spot at present, if visible. This proves that 

 the neighbourhood of the red spot had followed the particular 

 motion of the spot itself. This place is well characteri-ed by 

 the permanent depression in the great reddish band of the 

 planet. A. Ricc6 



Royal Observatory, Palermo, September 10 



"Elevation and Subsidence" 



Mr. O. Fisher has been so good as to offer a reply to my 

 "remark with a query," his answer being (allowing for an 

 obvious printer's error) that it is "an open question whether the 

 melting temperature of rocky matter is, or is not, raised by 

 pressure." 



I cannot for a moment pretend to the same familiarity with 

 the results either of experiment or of calculation as is doubtless 

 possessed by Mr. Fisher. I only claim to speak as representing 

 the class whose knowledge on these subjects is essentially second- 

 hand ; but, speaking as such, I think that Mr. Fisher's reply "ill 

 not generally be regarded as satisfactory. I should, therefore, 

 like to repeat my question with a little extension : — 



1. Do not the "rigidity" calculations incontestably show that 

 the earth is extremely rigid, i.e. solid ? Are not, therefore, all 

 theories which disregard this result (such as that the nucleus may 

 be above its own critical temperature) put out of count ? 



2. Are not the phenomena of metamorphic and hypogene 

 rocks on too large a scale to be accounted for by heat of merely 

 local origin, whether produced by chemical or mechanical action, 

 such as has been suggested in connection with volcanoes ? 



3. Do not all reasonable views of the origin of the earth, i.e. 

 any form of the nebular hypothesis, point to the same conclusion 

 as (2), viz. that the earth's heat is the residuum of a much 

 greater amount formerly possessed, and not yet entirely lost by 

 radiation ? 



4. Does not (3), taken in connection with the known laws of 

 conduction, involve a continuous increase of temperature, whether 

 rapid or flow, as we descend below the surface? 



5. Although we may have no direct evidence as to the 

 "temperature at depths bearing considerable ratios to the 

 radius," is there not ample evidence that at comparatively in- 

 significant depths the temperature is such as would melt not 

 only "rocky matter," but far more refractory substances, if 

 there were no counteracting influence ? Even allowing a very 

 slow increase, provided the increise is always positive, as 4 

 points out, should we not sooner or later almost certainly reach 

 the melting temperature of the most refractory substances 

 with which we are acquainted ? 



6. Can we then escape the conclusion, either that the nucleus 

 consists of matter of a totally different kind from anything with 

 which we are familiar, or that pressure raises its melting tem- 

 perature ? But does not every fact bearing on the question dis- 

 credit the former hypothesis ? 



7. Should we not then accept the view that pressure does 

 raise the melting-point of nucleus stuff, at least as a working 

 hypothesis, only to be overthrown by direct evidence to the 

 contrary, if direct evidence on the subject is ever forthcoming ? 



Trinity College, Cambridge F. VouNG 



In a paper I read before a full meeting of the Geological 

 Association on March 2 last, of which a brief notice is given in 

 Nature, vol. xxvii. p. 523, I discussed the probability of sub- 

 sidence of land, in certain cases, being due to loading by local 

 accumulations of terrestrial matter acting upon a deflectible crust 

 supported upon a viscous interior. The greatest effects, I ima- 

 gined, from this cause, were due to local accumulations of ice past 

 and present, particularly about the poles of the earth ; but that 

 secondary and important effects were due to the weight of accu- 

 mulations of solid mineral matter from denudations being carried 

 by oceanic currents and winds, from coral deposition, and the 

 reaction of volcanic outflows. One illustration I proposed was 

 that the sinking of the coast of Greenland was probably due to 

 the weight of inland accumulation of ice, which proposition I 

 thought was original, but Mr. Gardner (Nature, vol. xxviii. 

 p. 324) says—" It has often been supposed that the sinking of 

 the coast of Greenland is similarly due to its icecap." I should 



feel obliged if Mr. Gardner would point out references where 

 this has been proposed, as I thought I had read the literature of 

 the subject, and I fear that this part of my paper is less original 

 than I assumed. W. F. Stanley 



That there is a connection between sedimentation and sub- 

 sidence on the one hand and bet" een denudation and elevation 

 on the other is a fact now admitted by mot geologists. The 

 real question to be answered, however, is :■ — Are these directly 

 connected as cause and effect ? or are they simply concomitant 

 effects of the same cause ? If the first be true, we should expect 

 cause and effect to vary together, that is, that subsidence should 

 keep an even pace with sedimentation. That this has not been 

 exceptionally the ca=e is proved by the sections of the car- 

 boniferous system in the central valley of Scotland, where the 

 facts point to a continuous subsidence, accompanied by a very 

 inegular sedimentation, with the result that now subsidence 

 gained on sedimentation, now sedimentation on subsidence. 

 Again, once the process commenced — and it is not very evident 

 ho v on an originally even surface it could have commenced at 

 all — we should expect it to be continuous. Sedimentation 

 causes subsidence, subsidence gives rise to fresh sedimentation, 

 and that again to renewed subsidence, and so on and on. Con- 

 sequently we should expect that when once an area of sedimen- 

 tation and subsidence was formed, it would continue an area of 

 sedimentation and subsidence through all geological time. 



It appears rather, I think, that the connection between them 

 arises from their being concomitant effects of lateral pressure in 

 the earth's crust (for notwithstanding the Rev. O. Fisher's mas- 

 terly exposition of the inadequacy of this cause to produce the 

 observed inequalities of the earth's surface, I still believe that, 

 with the exception of the ocean basins, which mu^t be otherwise 

 accounted for, it is quite competent to account for the facts). 

 We may suppose the action to take place so : — 



A certain portion of the earth's crust is first thickened and 

 strengthened by volcanic outburst or other accumulation on the 

 surface. This part, when the tangential thrust comes, offers, by 

 reason of its increased weight and thickness, a greater resistance 

 to the elevating force than the parts around, and as a consequence 

 these are raised around the thickened part, while it is at the same 

 time depressed in a corresponding degree ; in other words it 

 becomes the centre of a syncline, while the strata around are 

 rai-ed into anticlines. Depression naturally leads to sedimenta- 

 tion, and this still more thickens the part, and enables it to offer 

 greater resistance to the tangential thrust, with the result that it 

 continues to be depressed as the strata around are elevated. The 

 converse is also true. Denudation means the thinning and con- 

 sequent weakening of the crust, and hence when the thrust comes 

 the denuded part is the more likely to be elevated into the 

 anticline. 



This theory provides for the cessation of the phenomena, since 

 the tension of the crust is after a time relieved. It also accounts 

 for the fact that strata around volcanoes and volcanic necks, as 

 also along the base of mountain chains, so frequently appear lo 

 dip below them. The rate of subsidence, too, would vary with 

 the intensity of the exciting force, though the consequent sedi- 

 mentation need not vary with it in the same absolute degree. 



Perth, September 3 William Mackie 



My article on elevation and subsidence has provoked con- 

 siderable and, on the whole, friendly criticism, a so far satisfac- 

 tory result, though but few points have been raised requiring reply. 

 Dr. Ricketts objects, and very properly, that I have not alluded 

 to his many writings on the subject ; and to this I can only plead 

 want of space, that I have not entered at all into its already 

 voluminous bibliography, and that my article was written and in 

 type before his recent contributions to the Geological Magazine 

 had appeared. Beyond this I had sufficiently indicated that there 

 were many observers in the field, and every geologist must be 

 aware that the subject has for a long while past excited attention 

 not only in England but in France and America. 



The fundamental error in my article is pointed out by the 

 Rev. Mr. Fisher and by Mr. Young, and the assumption that 

 inert pressure induces heat must be abandoned. As I had read 

 the " Physics of the Earth's Crust," I expected that this would 

 be challenged, but I let it stand, as the fallacy has been shared 

 by a large number of geologi-ts, comprising some of the most 

 distinguished, and has even escaped the correction of physicists. 

 But this rectification, while very important, by no means affects 

 the results, and on the contrary facilitates an appreciation of the 



