488 



NATURE 



[March 22, 1888 



by the phrase "a force equal to the weight of lo pounds," which 

 is neither clumsy nor absurd. 



(10) "Except for the parts criticized above, on the units of 

 weight, mass, and force, the present treatise shows that the 

 author has read with profit and discrimination the most recent 

 treatises on dynamics." I have been under the impression that 

 in my treatment of these units I had, in the main, followed the 

 most recent treatises on dynamics. May I ask in which of them 

 units are treated in what Prof. Greenhill considers the proper 

 way? 



I would like to say also that the elementary proofs of the 

 chief properties of the common catenary, which are given by 'me, 

 are, with slight modifications, those given in Prof. Goodeve's 

 " Principles of Mechanics." My indebtedness to his book is 

 acknowledged generally in the preface. 



I fear my desire to be brief may have made me appear curt. 

 Let meex^press, therefore, my appreciation of the trouble Prof. 

 Greenhill has taken to form a just estimate of the merits of my 

 book, and of the kindly way in which he has spoken of it. 



J. G. MacGregor. 



Dalhousie College, Halifax, N.S., March i. 



;_Coral Formations. 



I AM glad to see the theory that the internal lagoons of coral 

 atolls are excavated by the chemical action of sea- water and the 

 removal of carbonate of lime in solution is now being brought to 

 the test of figures. 



Mr. J. G. Ross (Nature, March 15, p. 462) calculates 

 from his experiments that in this way a sheet of carbonate of 

 calcium half an inch thick can be removed annually from the 

 surface of a lagoon, but strangely adds, " In other words at the 

 same rate it would require about a century to deepen the lagoon 

 one fathom." According to this method of calculating, 144 years 

 is "about a century," 



These figures no doubt suit the theory of the formation of 

 coral lagoons very well, but they appear to me quite destructive 

 of the other and co-relative view that the platforms upon which 

 atolls have been formed have been built up by the accretion of 

 the dead shells of pelagic organisms showered down from the 

 surface of the ocean together with the shells of those organisms 

 which have lived on the bottom. I believe that at no place on 

 the surface of the globe are such dead shells being supplied at a 

 rate that would even balance this supposed rate of chemical 

 destruction. 



Yet if these figures be correct we shall have to reckon upon 

 the removal from such platforms of more than half an inch 

 annually in consequence of the quicker action which it is said 

 takes place through greater pressure at greater depths. 



If, therefore, we accept the dissolution theory of the origin of 

 coral lagoons, it seems impossible to believe in the building up 

 of platforms of calcium carbonate on volcanic or oth;r peaks 

 from varying and unknown depths to the levels necessary for the 

 growth of reef corals. If, on the other hand, we believe that 

 platforms are so built up, it appears equally destructive of the 

 dissolution theory of the lagoons. 



Dr. Darwin indicated this difficulty in his letter to me, 

 published in Nature, November 17, 1887, p. 54, but the 

 figures we are now supplied with enable us to realize it ^much 

 more vividly. T. Mellard Reade. 



Park Corner, Blimdellsands, March 16. 



The Movements of Scree-Material. 



I PERUSED with interest the abstract of a paper on the above, 

 read by Mr. Davison at the meeting of the Geological Society 

 on the 29th ult. 



The phenomenon seems somewhat akin to the movements in 

 the "Stone Rivers " of the Falkland Islands, though another 

 reason has been suggested by Sir Wy ville Thomson as the cause 

 of their progress. 



Might it not be possible for motion to be produced in loose 

 materials, and in the molecules of certain coherent sub.stances 

 situated at a high angle of slope, bycontinual though imperceptible 

 vibrations in the earth's crust ? 



Apart from the changes wrought by alternating temperature, 

 might not the "downward creep" in the lead on the roof of 

 Bristol Cathedral — a» observed by Cr.non Moseley — be due to 



a " settling down " of the molecules by the constant vibrations 

 of sounds transmitted through the structure, and having their 

 origin within and without? Cecil Carus-Wilson. 



iiournemouth, March 15. 



Were the Elephant and Mastodon contemporary 

 in Europe ? 



Mr. Howorth asks this question in Nature for March 15 (p. 

 463). Perhaps this extract from a translation of a note from Prof. 

 d'Ancona, of Florence, will satisfy Mr. Howorth : "The soil of 

 the upper Val d'Arno is ascribed to formations of the Pliocene 

 period." In it have been found ^'Mastodon avcrnensis, Elephas 

 mcridionalis." Twenty-four other animal remains'are identified, 

 all differing from the remains of the bone-caves. In both places 

 respectively these relics belong to contemporary animals. 



9 Sinclair Road, W., March 15, H. P. Malet. 



EXPERIMENTS IN MOUNTAIN BUILDING} 



THE primary object of these experiments was to 

 explain on what mechanical principles the remark- 

 able rock-structures recently discovered by the Geo- 

 logical Survey in the North-West Highlands might have 

 been produced. In experimenting on the behaviour of 

 strata when subjected to horizontal pressure, it has been 

 usual to regard large rock-masses as practically plastic 

 bodies, and to imitate in the laboratory the great flexures 

 and plications of Nature by coinpressing layers of clay, 

 cloth, and other plastic or iiexible substances. It was, how- 

 ever, evident, as soon as the true structure of the North- 

 West Highland area was unravelled, that the rocks had, 

 to a very large extent, behaved like rigid bodies under 

 the enormous lateral pressure to which they had once 

 been subjected. Instead of following the usual method 

 of using plastic materials, the author therefore set to 

 work to devise strata sufficiently rigid to snap rather than 

 bend and become folded on the application of lateral 

 pressure. It is to this peculiarity in the character of the 

 materials, rather than to any great novelty in the methods, 

 that the interesting results obtained are mainly due. 



The experiments were of three distinct kinds. The 

 first series was designed to explain the behaviour of strata 

 when thrust horizontally over an immovable surface, and 

 thus to throw light on the phenomena of " thrust planes," 

 such as are now known to occur abundantly in the North- 

 West Highlands between Loch Eriboll and Skye (see 

 Naturk, vol. xxxi. p. 33). To simulate natural strata, 

 layers of damp sand, foundry loam, or in a few cases 

 clay, with laminae of dry stucco powder between, were 

 employed. In a few minutes the anhydrous powder 

 absorbed enough moisture from the damp beds to enable 

 it to " set " into tolerably rigid sheets. The rock which 

 had thus solidified i7t situ, was next compressed hori- 

 zontally, by pushing in, by hand, or with the help of a 

 screw, the movable end of the long box in which the 

 strata were formed. One side of the box could be re- 

 moved at pleasure, and at the end of each experiment it 

 was lifted off, and the section inside revealed, so that it 

 could be photographed or copied if desired. 



Fig. I, which is drawn to a scale of j^.t of the original, 

 shows the character of the section produced after the 

 end had been pressed in 20 inches. The central light- 

 coloured band, bounded by stiff stucco lamina:;, has under- 

 gone no folding, but has become heaped up by means of 

 a series of slightly inclined reversed faults, along which 

 the constant pressure from the right found relief. For this 

 structure the author has proposed the name "wedge 

 structure," as the advancing mass is really raised by being 

 forced over a series of wedges of undisturbed rock. 



After pushing the piled-up mass a certain distance 



' Abstract of a Paper by Henry M. Cadell, B.Sc, F.R.S.E., H.M. 

 Geological Survey of Scotland, read before the Royal Society of Edinburgh, 

 February 20, 1888. 



