July 5, 1900J 



NATURE 



237 



is obtained little short of the desired 3 '61- This result must 

 appear sufficiently striking, but it may be seriously doubted 

 whether even an approximate estimate of the total bulk of sedi- 

 mentary strata can possibly be arrived at. Such an estimate must 

 inevitably rest in great part on a basis of pure speculation. Not 

 only are we ignorant, as regards huge areas, of the thickness of these 

 strata, but immense tracts still remain unexplored so far as their 

 geology is concerned. Further, the boldest guess can tell us 

 little of sedimentary strata hidden beneath the surface of the 

 ocean, and it may be looked upon as a lucky coincidence that 

 Prof Joly is able to attain the above result when restoring to 

 the estimated sedimentary rocks the sodium of the sea. The 

 question also of pre-Cambrian rocks of sedimentary origin 

 appears here to be too lightly passed over, for although so little 

 is known of their actual extent, the trend of recent researches 

 has been to show that they may constitute a not unimportant 

 fraction of the total sediments formed. It is scarcely necessary 

 to recall the fact that the earliest known fossil faunas, including 

 marine forms of comparatively high organisation, clearly indi- 

 cate that a habitable ocean had already for long ages been in 

 existence. 



The unequal ratios of the alkalies in the ocean and in the 

 rivers respectively next receive attention. The fact that the 

 ratio of potash to soda is very much higher in the rivers than in 

 the sea, is believed by the author, not to indicate that the rivers 

 now contain more potash relatively to soda than in former times, 

 but is to be accounted for by the constant abstraction of potash 

 from the ocean, largely in the glauconite now forming on the 

 sea floor, and so extensively distributed in the sedimentary 

 strata. Stress is also laid on the fact that potassium brought 

 from the atmosphere by rain tends to become retained upon the 

 land, while the sodium is more readily returned to the ocean. 

 In arguing for the uniformity of denudation by solution in past 

 times, Prof. Joly brings forward some good reasons to show 

 that the distribution of land and sea can have varied but little. 

 As regards the greater exposure of igneous rocks in early times 

 some interesting points on the nature of weathering and soil 

 formation are noted, and it is concluded that the unequal per- 

 centage of sodium in the igneous and sedimentary rocks would, 

 as regards supply to the ocean, be counterbalanced by the 

 different rates of weathering. Sedimentary rocks, poorer in 

 alkalies, allow of more rapid denudation. 



In the concluding section of this paper the action of the 

 ocean as an agent in solvent denudation is dealt with. Such 

 action, the author maintains, is carried on chiefly along the 

 coast lines, and is very small as compared with that effected by 

 rain and river waters. Experiments are quoted to show that 

 the power of sea-water to decompose felspar is minute in com- 

 parison with that exerted by fresh water. It is further pointed 

 out that the volcanic debris of oceanic deposits have the alkali 

 ratio of igneous and not that of sedimentary rocks. A correction 

 of half a million years on the original time estimate is thought 

 to be a sufficient allowance to make for the solvent denudation 

 by the ocean. But even allowing, as held by the author, that 

 chlorides other than sodium chloride may in past times have in 

 some measure retarded solvent denudation by the ocean, it may 

 be suggested that subaqueous volcanic action, at one time more 

 frequent than at present, with its attendant conditions of excep- 

 tional temperature and pressure, may by frequent repetition 

 through vast periods of time have played some part in aiding 

 this process. 



Prof. Joly does well in finally recognising the uncertainty 

 attending his corrections on the original estimate of geological 

 time, and he certainly allows no too wide a margin for error in 

 the final result when he claims that " a period of between 

 eighty and ninety millions of years " has elapsed since the land 

 first became exposed to denuding agencies. For not only in 

 the data upon which the corrections are founded, but also in the 

 factors employed in the original calculation, there is to be found 

 comparatively little of certainty and much that is purely specu- 

 lative. In this latter category must be placed the supposed 

 sequence of events at the first cooling of the globe. The rela- 

 tive intensity of geological activities in the past is also unknown 

 to us, and the possibilities as regards the activity of the sun and 

 the influence of the moon in modifying meteorological agencies 

 during the earlier chapters of the earth's history appear to 

 render hopeless the final solution of the lime-problem by such a 

 method as that here employed. But in this interesting treatise 

 Prof Joly has with marked ability and originality attacked a 

 most difficult question, and his novel theory calls for the fullest 

 consideration from all geologists and physicists. 

 NO. 1601, VOL. 62] 



NOTES ON SATURN AND HIS MARKINGS. 



'T'HE possessors of telescopes will welcome the reappearance 

 ■^ of Saturn as a rather conspicuous object in the evening 

 sky. The planet now rises at 7h. 40m. p.m., and remains 

 visible afterwards throughout the night, but unfortunately his 

 altitude is extremely low. His southern declination being 22^, 

 his position is only 16° above the horizon at Greenwich even at 

 the time of his meridian passage. Notwithstanding these un- 

 favourable conditions, excellent views may, however, occasionally 

 be obtained of his general aspect. From stations in the southern 

 hemisphere the planet may be seen under the best circumstances. 

 This planet with his rings, belts and moons, forms a picture 

 quite unique of its kind. The globe is greatly compressed at 

 the poles, like that of Jupiter, and the rate of its axial rotation 

 similarly rapid. We recognise also in the dusky bands of Saturn 

 another parallel to the visible lineaments of the " Giant Planet,'* 

 but there is a marked difference as regards the distinctness with* 

 which the details on the two bodies may be viewed. JupilerV 

 large disc and superior brilliancy enable the markings suxi' their 

 variations of form and motion to be followed with great facility- 

 and certainty. Saturn being much smaller andfamteris more 

 difficult, especially as regards the more deHeate features... 

 Cassini's division in the rings and the principal belt on the gl'obe 

 may be distinguished with a two inch refractor, but Encke's 

 division in the outer ring is a doubtful, or probably a very variable 

 feature, which at certain times appears to be missir^ altogether, 

 while on other occasions it is described as faintly outlined as a 

 pencil-like curve at the ansce. 



That there are occasional irregularities on Saturn is proved 

 beyond contention. In 1790 Sir W. Herschel remarked a 

 very dark spot on the limb, and in 1793 noticed some irregu- 

 larities in a quintuple belt which enabled him to ascejitain' the: 

 planet's rotation period. The lai^e white spot seer? by PVcut 

 Hall and others at the close of 1876 affords a good instance of 

 change, and it is well-known that the disposition and number 

 of the belts vary from year to year. We naturally conclude 

 that these belts must occasionally exhibit irregularities likQ 

 those of Jupiter. 



The planet is now presented to us at an angle which permits 

 the ring system to be seen with splendid effect. We now view 

 the northern side of the ball and rings, and this will continue 

 to be the case until 1907. 



Perhaps there is no object upon which it is easier to exercise 

 the imagination than upon Saturn. And there is probably no% 

 orb in reference to which more errors in detail have been made,, 

 though both Mars and Venus have encouraged a large rrumber 

 of observational misconceptions. Many of the abnormal results- 

 reported in recent years, and due to small instruments, may be 

 safely dismissed, for they are not only doubtful but, when all 

 the conditions are considered, ridiculous, and palpably the out- 

 come of unconscious suggestions of the imagination. Yet there 

 can be no question as to the good fiiith of those who are re- 

 sponsible for some of the wonderful seeings lately published. 

 They honestly believe they have seen what they have drawn, 

 and as a matter of fact it is an extremely difficult point to-- 

 distinguish between real and imaginative features on Saturn. 

 The trembling of the image, its faintness under high power or 

 its smallness under low power encourage much fictitious detail, 

 which every observer cannot regard as illusory. 



Some of those who claim to have seen many irregular 

 markings on this beautiful planet ascribe their success to- 

 special training; but this explanation will scarcely stand, fo» 

 others of equal experience and using more powerful appliances 

 have quite tailed to observe them. The difference is not one 

 of sight, of practice, or of instrumental means. It resolves it- 

 self into a question of personal ethics. There are men who will 

 report nothing but what they are absolutely certain is presented 

 to their eyes, and are unbending in their regard for the truth ; 

 there are others who, though equally sincere in intention, are 

 not so reliable in their judgment, and accept features which are- 

 apparently glimpsed, but which are in reality prompted by the 

 imagination on an unsteady and very delicate object. 



It is to be hoped that time will eliminate all the fanciful 

 representations of Saturn which recent observations have sa 

 abundantly supplied. The period has now arrived when the 

 planet may be telescopically surveyed with a view to obtain a 

 really sound knowledge of such features as are portrayed in 

 moderately powerful instruments. Those who have employed 

 large and small telescopes in planetary observation aver that the 

 former are more effective than the latter ; but it is remarkabl* 



