Sept. 28, 1876] 



NATURE 



481 



cliff line, similar to the little mounds left by the sea at the pre- 

 sent day half-way between the mark of the highest spring tides 

 and that of the lowest neap tides. With regard to the date of 

 the elevation of the beaches, the authors observed that all the 

 evidence available tended to place it before the time of the 

 Roman occupation. 



Tidal Retardation — Argument for the Age of the Earth. — The 

 Secretary read a paper by James Croll, LL.D., F.R.S., of the 

 Geological Survey of Scotland, On the Tidal Retardation 

 Argument for the Age of the Earth. Many years ago Sir 

 William Thomson demonstrated from physical considerations 

 that the views which then prevailed in regard to geological 

 time and the age of our globe were perfectly erroneous. 

 His two main arguments were — first, that based on the sun's 

 possible age ; and secondly, that based on the secular cool- 

 ing of the earth. More recently he has advanced a third 

 argument {Trans. Geol. Soc. of Glasgow, vol, iii., p. i), based 

 on tidal retardation. It is well known that owing to tidal retar- 

 dation the rate of the earth's rotation is slowly diminishing, and 

 it is therefore evident that if we go back for many millions of 

 years we reach a period when the earth must have been rotating 

 much faster than now. Sir William's argument is, that had the 

 earth solidified several hundred millions of years ago the flatten- 

 ing at the poles and the bulging at the equator would have been 

 much greater than we find theui to be. Therefore, because the 

 earth is so little flattened it must have been rotating, when it 

 became solid, at veiy nearly the same rate as at present. And 

 as the rate of rotation is becoming slower and slower, it cannot 

 be so many millions of years back since solidification took place. 

 A few years ago I ventured to point out (Nature, August 21, 

 1871 ; "Climate and Time," p. 335) what appeared to be a 

 very obvious objection to the argument, and as the validity of 

 the objection, as far as I am aware, has never been questioned, I 

 have been induced to believe that the argument referred to had 

 been abandoned. But I find that Prof. Tait in his work on 

 " Recent Advances in Physical Science," restates the argument 

 as perfectly conclusive, and makes no reference whatever to my 

 objection. As the subject is one of very considerable import- 

 ance, I may be permitted to direct attention to the objection in 

 question, which briefly is as follows : — 



It has been proved by a method pointed out a few years ago 

 {Philosophical Magazine, May, 1868, pp. 378-384, February, 

 1867, p. 130, " Climate and Time," Chap. xx. Transactions 

 of Geological Society of Glasgow, vol. iii., p, 153), and which is 

 now generally admitted to be reliable, that the rocky surface 

 of our globe is being lowered, on an average, by subserial denu- 

 dation at the rate of about i foot in 6,000 years. It follows as 

 a consequence from the loss of centrifugal force resulting from 

 the retardation of the earth's rotation, occasioned by the friction 

 of the tidal wave, that the sea-level must be slowly sinking at the 

 equator and rising at the poles. This, of course, tends to pro- 

 tect the polar regions, and expose equatorial regions to subaerial 

 denudation. Now it is perfectly obvious that unless the sea- 

 level at the equator has, in consequence of tidal retardaion, 

 been sinking during past ages at a greater rate than I foot in 

 6,000 years, it is physically impossible the form of our globe 

 could have been very much different from what it is at present, 

 whatever may have been its form when it consolidated, because 

 subserial denudation would have lowered the equator as rapidly 

 as the sea sank. But in equatorial regions the rate of denudation 

 is, no doubt, much greater than in the temperate regions. It has 

 been shown in the papers above referred to, that the rate at which 

 a country is being lowered by subaerial denudation is mainly deter- 

 mined not so much by the character of its rocks as by the sedi- 

 mentary carrying power of its river systems. Consequently, 

 other things being equal, the greater the rain-fall the greater will 

 be the rate of denudation. We know that the basin of the 

 Ganges, for example, is being lowered by denudation at the rate 

 of about I foot in 2,300 years, and this is probably not very far 

 from the average rate at which the equatorial regions are being 

 denuded. It is therefore evident that sub-Derial denudation is 

 lowering the equator as rapidly as the sea-level is sinking from 

 loss of rotation, and that consequently we cannot infer from the 

 present form of our globe what was its form when it solidified. 

 In as far as tidal retardation can show to the contrary, its form 

 may have been as oblate as that of the plant Jupiter when soli- 

 dification took place. 



There is another circumstance which must be taken into 

 accoimt. The lowering of the equator by the transference of 

 materials from the equator to the higher latitudes must tend to 



increase the rate of rotation, or, more properly, it must tend to 

 lessen the rate of tidal retardation. 



On Siliceous Sponges from Carbonijerous Limestone near Glas- 

 gow, by John Young, F.G. S. — Mr. Young observed that sili- 

 ceous sponges had not hitherto been obtained from deposits of 

 Carboniferous Limestone age in Britain. Recently, however, 

 Mr. John Smith had discovered large numbers of them in 

 fissures in a limestone at Cunningham Baidland, neir Dairy, 

 Ayrshire. The limestone bed in which they occur is 40 feet 

 thick, and belongs to the upper division of the Carboniferous 

 Limestone series. It contains, at different horizons, producd 

 and spirifers, corals, crinoids, and polyzoa. Prof. Young and 

 the author proposed to name them Acanthaspongta Smithii. 



On the Granite of Strath-Errick, Loch Ness, by James Bryce, 

 LL. D. — Having ascertained that the gold of Sutherland occurred 

 not in quarts veins, but in the granite itself, Dr. Bryce tested the 

 granite of Strath-Errick, and was rewarded by finding gold there 

 also, although in small quantities. Proceeding to examine more 

 carefully than had previously been done the relations of this 

 granitic mass to the surrounding rocks, he found that, although 

 at one locality it clearly overlaid the Lower Old Red Sandstone, 

 in another place it alternated with slate, as if the slate had been 

 brought up by the granite. It was remarkable that, although 

 the slates are cut up by veins of the granite, none pass into the 

 Old Red strata. The author considered that the evidence in 

 favour of the intrusive character of the granite was incontro- 

 vertible. 



On the Upper Silurian Rocks of Lesmahag07v, by Dr. Robert 

 Slimon. — Dr. Slimon gave an interesting historical account of 

 the mapping of the Upper Silurian rocks of Lesmahagow, and 

 of the discovery and determination of their remarkable crustacean 

 fauna. 



On the Age, Fauna, and Mode of Occurrence of the Phosphorite 

 Deposits of the South of France, by J. E. Taylor, F.G.S. — The 

 author visited the phosphorite caverns within the last two months, 

 and gave an account of what he saw. 



On a Deep Boring for Ccal at Scarle, Lincolnshire, by Prof. 

 E. Hull, M.A., F.R. S. — The boring, after penetrating the 

 Lower Lias, New Red, and Permian, entered the Carboniferous 

 formation at the depth of 1,900 feet. The Carboniferous Rocks 

 bored through were grey sandstones, with plants and shales with 

 anthracosia, &c., 55 feet ; calcareous shales and earthy limestone, 

 65 feet ; fine breccia, 4 feet ; chocolate-coloured clay, 6 feet. 

 This succession was very puzzling. The beds above the breccia 

 were pronounced by Prof. Ramsay and the author, without any 

 consultation, to be Yoredale Rocks, but since the breccia has 

 been reached. Prof. Hull inclines to regard it as belonging to the 

 uppermost beds of the Coal Measures. As the boring is still 

 going on, it is hoped that something more definite may be dis- 

 covered. 



A feeder of water was tapped in the Keuper Sandstone at the 

 depth of 917 feet, and a still more powerlul one in the Bunter 

 Sandstone, at 1,250 feet, sent a jet of clear water 4 feet above 

 the ground. The water must percolate from the outcrop of 

 these beds ten or twelve miles to the west, being prevented 

 from rising by the presence of the overlying impervious Lias 

 Clay. 



On Tertiary Basaltic Dykes in Scotland, by R. L. Jack, 

 F.G.S., of the Geological Survey of Scotland. — Mr. Jack exhi- 

 bited a map showing the courses of all the dykes of this age 

 traceable for any distance which have hitherto been mapped by 

 the Geological Survey, and described their peculiarities, referring 

 specially to their avoidance of faults and other obvious lines of 

 weakness. One dyke crosses Scotland from Helensburgh to 

 Grangemouth, while two others maintain a parallel course from 

 the heads of the River Irvine to the head of the Tweed, a dis- 

 tance of nearly forty miles. It was pointed out that a number 

 of the larger dykes tend to converge towards the peninsula 

 between Lochs Riddun and Striven, where, however, no evi- 

 dence of volcanic activity, either in the shape of lava-flows or 

 plugged-up vents, is known to exist 



On certain Pre- Carboniferous and Metamorphosed Trap-Dykes 

 and Associated Rocks in North Mayo, by W. A. Traill, M.R.I. A., 

 of the Geological Survey of Ireland. — In the district between 

 Ballycastle and Belmullet the rocks belong either to the Car- 

 boniferous age or are older and metamorphosed. The author 

 distinguishes at least two sets of dykes, both being basaltic. 

 Those of the newer set run in straight lines, traverse both meta- 

 morphic and Carboniferous strata, and appear to fill vertical 

 fissures or to come up along lines of fault. The older dykes 



