148 



NATURE 



[June 12, 1890 



the meaning" of what I wrote, I must leave it to others to settle 

 whether this be in a favourable or an unfavourable sense. 



Turning to another matter : in reference to an interesting 

 paragraph on p. i lo concerning the excavation of rock by snails, 

 a subject on which I once -wrott {Geol. Afag., 1869, 1870), may I 

 ask whether any of the readers of Nature are acquainted with 

 instances of these burrows occurring in non-calcareous rocks ? 

 All which I have seen were in limestone, and, as I believe, 

 always in a pure variety. Hence, in the case of snails, one 

 would suspect that the excavation was mainly due to chemical 

 action. T. G. Bonney. 



Coral Reefs, Fossil and Recent, 



I SUPPOSE it will be expected of me that I should answer the 

 two objections raised by Captain Wharton (May 22, p. 81), viz. 

 (i) that he knows of no steep submarine reef-slopes exceeding 

 4000 feet in height ; and (2) that the lagoons could not be so 

 shallow as they are if we assumed any extensive positive shifting 

 of the coast-line. 



From the statements in the literature on the subject, concern- 

 ing point (l), I select the following three : — 



Captain Fitzroy found at the Keeling no bottom 2200 yards 

 from the breakers with a line 7200 feet long (Darwin, " Coral 

 Reefs "). 



Bourne says in his account of Diego Garcia (Proc. Roy. Soc. , 

 vol. xliii.), that the Maldives, Laccadives, and the Chagos 

 rise from a bank 1000 fathoms below the surface very abruptly. 



Heilprin (" Bermudas ") states that the Bermudas rise abruptly 

 out of a depth of 12,000 to 13,000 feet. 



Concerning point (2), I cannot see why the gentle inward 

 slopes of atolls should not be in harmony with the subsidence 

 theory. It must be borne in mind that the shifting of the coast- 

 line is both slow and oscillating. Positive and negative shift- 

 ings alternate. The latter predominates on the whole. Dr. 

 Murray says that in shallow water the accumulation of material 

 exceeds the removal by solution. I have professed my accord- 

 ance with this view in my previous letter. Particularly in an 

 inclosed or partially inclosed lagoon, sheltered from ocean 

 currents, this fiUing-up process will be a rapid one. We can 

 easily conceive that it will balance the subsidence until the 

 lagoon becomes so shallow as to impede the life of those 

 organisms whose skeletons form the raising-up deposit. If there 

 is any oscillatory negative shifting of coast-line, the dry rim will 

 rise, and extend horizontally, and afford to the atmospheric 

 agencies a larger surface wherefrom material can be washed into 

 the lagoon. 



On the whole, if there is anything difficult to explain, it is 

 that the lagoons are as deep as they are. Deep lagoons are, 

 however, not common, and are generally only met with in large 

 and interrupted atolls. Perfectly dry central depressions (with 

 deposits of gypsum and the like) are by no means infrequent in 

 very small atolls. The general proportionality of the depth and 

 the horizontal extent of the lagoons is perfectly in accordance 

 with the subsidence theory. It supports no other theory better 

 than this one. R. voN Lendenfeld. 



Photographs of Water Drops. 



In Nature of May 22 (p. 95) there is an account given of 

 the discussion following Mr. C. V. Boys's demonstration of his 

 photographs of falling water drops at the meeting of the Physical 

 Society. In the course of this discussion. Lord Rayleigh, who 

 was naturally much interested in the subject, remarked that it 

 had never occurred to him that it would be possible to get 

 enough light from a single spark to photograph the drops as 

 Mr. Boys had done. And Lord Rayleigh believed Mr. Boys's 

 success was owing to the fact of his using no lenses, which would 

 absorb the ultra-violet rays. 



With reference to this, it might, perhaps, be interesting to 

 mention that I succeeded very well, some years ago, in photo- 

 graphing water drops, falling through air, with single sparks, the 

 light of the spark passing two glass lenses and the objective of a 

 camera which gave magnified images. My photographs (copies 

 of which appeared in the Atinalen der Physik unci Cheviie, vol. 

 xxx., 1887) show all the forms obtained so very beautifully by 

 Mr. Bojs. From photographs taken at different depths below 

 the orifice of the tube I could measure the periodic time of the 

 elliptical vibrations and of the vibrations according to the next 

 higher spherical harmonic, and show that the ratio of these two 



NO. 1076, VOL. 42] 



periodic times agreed very closely with the formulae given by 

 Lord Rayleigh in the Proc. Roy. Soc, 1879. The amplitudes 

 had no influence upon the periodic times. 



Richmond, Surrey, June 6. P. Lenard. 



THE CLIMATES OF PAST AGESy 

 I. 



T T happens sometimes in the history of science that a 



•»■ few striking facts lead to the building up of a far- 

 reaching theory, which at first satisfies us, and with 



which, without being rigorously critical, we endeavour to 

 bring the further results of experience into conformity. 



But contradictions and difficulties gradually manifest 

 themselves, and go on accumulating, until at last we are 

 convinced that we have built on an unsure foundation, 

 and that the edifice that we have raised upon it must be 

 utterly pulled down. Then follows a period of discussion 

 and collection of further evidence, during which we ab- 

 stain from any attempt to substitute new and more 

 correct explanation for that which we have abandoned, 

 until by assiduous labour we shall have prepared a 

 broader and more stable basis for the superstructure. 



In such a stage of transition, the old ground aban- 

 doned, the new not yet won, is our knowledge of the 

 climatic conditions of our earth in bygone ages. In the 

 far north a rich mass of fossilized plants and coal-beds 

 had been found in the Carboniferous formation. Reef- 

 building corals, such as to-day live only in tropical seas, 

 were yielded by the Carboniferous limestone and the 

 Silurian formation up to 80° of northern latitude ; and many 

 of the species were found to range, without any essential 

 change of form, from arctic to temperate, nay in some 

 cases even to equatorial regions. From a small number 

 of data such as these it was hastily concluded that, under 

 the influence of the internal heat of the earth, a warm 

 uniform climate must have prevailed generally from the 

 pole to the equator, while a sultry atmosphere, heavily 

 charged with water vapour and carbonic acid, prevented 

 the sun's rays from reaching the earth or in any case from 

 exercising any considerable influence on it. As a conse- 

 quence, the existence of climatic zones or of a distribu- 

 tion of the fauna and flora in such zones was denied. It 

 was held that with the beginning of the Tertiary era a 

 polar cooling first set in, and that it increased during 

 its passage, until the present distribution of heat was 

 brought about as the final result of this long-continued 

 process. 



The falsity of these assumptions is now pretty generally 

 recognized, and the number of their adherents diminishes 

 daily. It would lead us too for afield were we to follow 

 out the hypothesis into all the details of its ofttimes fan- 

 tastic errors, and to note their individual failure. It will 

 be more to the purpose if, in the first place, we test the 

 methods by which we arrive at conclusions on the tem- 

 perature conditions of past ages, in order that we may 

 thus gain a knowledge of what these really were and of 

 the better-grounded attempts to explain them. 



Among the more important data for judging of the 

 climate of a past epoch, is the character of its plants and 

 animals, on the assumption that these various organisms 

 must have lived under nearly the same conditions of 

 temperature as their nearest relatives now existing. This 

 kind of reasoning has been very extensively applied, and 

 within certain limits its validity cannot be gainsaid. If, 

 for instance, in a comparatively recent deposit of the 

 Pleistocene period in Central Europe, we find remains of 

 the arctic willow, the dwarf birch, the white dryas, together 

 with such mammals as the lemming, the musk-ox, the 



' Translation of a Lecture delivered by the late Dr. M. Neumayr before 

 the Society for the Dissemination of Natural Science, at Vienna, on January 



2, 1889. 



