July 19, 1900] 



NA TUBE 



27s 



one studying the subject from a scientific as well as economic 

 point of view. Mention is made of beds of white pulverulent 

 silica, which when mixed with clay has been used in the 

 manufacture of a paint. 



We have received from the Geological Survey of Canada, Part i 

 of a " Catalogue of Canadian Birds," by Mr. J. Macoren, 

 dealing with water-birds, gallinaceous birds, and pigeons. 



The third volume of Prof. G. O. Sars's "Account of the 

 Crustacea of Norway," dealing with the anomalous group 

 Cumacea, is in course of publication by the Bergen Museum. 

 Parts V. and vi., devoted to the Diastylidse, have just been 

 issued. 



Part io of Memoir III. of the Australian Museum, Sydney, 

 on "The Atoll of Funafuti" has now been issued. It is the 

 concluding part of the memoir, and contains lists of the 

 contributors and plates, and an index to the whole work. 



Messrs. Isenthal and Co., have issued a revised edition 

 of their list of apparatus and accessories for work with Rontgen 

 rays. Particular attention is given by this firm to the design 

 and construction of instruments for radiographic work, and any 

 one contemplating an installation for this purpose will find the list 

 just issued well worth examination. 



The additions to the Zoological Society's Gardens during the 

 past' week include a Patas Monkey {Cercopithecus patas, 9 ) from 

 West Africa, presented by Mr. W. B. Davidson Houston ; a 

 Rhesus Monkey {Macacus rhesus) from India, presented by Mrs. 

 Heigham ; a Common Marmoset {Hapale jacckus) from South- 

 east Brazil, presented by Mrs. Alexander Grant ; two Grey- 

 headed Love-Birds {Agapornis cana) from Madagascar, pre- 

 sented by Mrs. Harry Blades; a Cuckoo (Cucuhis canorus), 

 European, presented by Mr. L. W. Wiglesworth ; an Entellus 



Monkey {Seinnopithecus entellus, 9 ) from India, a Bear 



(Ursus, sp. inc.) from Kuldja, a Himalayan Snow Partridge 

 ( Tetrcgallus himalayetisis) from the Himalayas, two Brazilian 

 Tortoises ( Testudo tabulata) from South America, deposited ; a 

 Sharp-nosed Badger {Meles leptorhynchtis) from China, a Rough 

 Fox {Cams riidis) from South America, purchased; a Little 

 Bittern {Ardetta minutd), European, received in exchange ; a 

 Brindled Gnu {Conmchoetes iaurina, 9 ), an Altai Deer {Cervtis 

 euslephaims), born in the Gardens. 



RHYTHMS AND GEOLOGIC TIME.^ 

 ■^HE subject to which I shall invite your attention this 

 evening is by no means novel, but might better be called 

 perennial or recurrent ; for the problem of our earth's age seems 

 to bear repeated solution without loss of vigour or prestige. It 

 has been a marked favourite, moreover, with presidents and 

 vice-presidents, retiring or otherwise, when called upon to ad- 

 dress assemblies whose fields of scientific interest are somewhat 

 diverse— for the reason, I imagine, that while the specialist 

 claims the problem as his peculiar theme of study, he feels that 

 other denizens of the planet in question may not lack interest 

 m the early lore of their estate. 



The difficulty of the problem inheres in the fact that it not 

 only transcends direct observation but demands the extrapola- 

 tion or extension of familiar physical laws and processes far 

 beyond the ordinary range of qualifying conditions. From 

 whatever side it is approached the way must be paved by pos- 

 tulates, and the resulting views are so discrepant that impartial 

 onlookers have come to be suspicious of these convenient and 

 inviting stepping stones. 



In giving brief consideration to each of the more important 

 ways by which the problem of the earth's age has been ap- 



' Abridged from an address to the American Association for the Ad- 

 rf ""%"' ^'^Scence at New York, June 26, by the retiring President, 



c 5^- Gilbert. By the courtesy of the Editor of iiciena 

 proofs of the address were received. 



NO. 1603, VOL. 62] 



Science, advance 



proached, I shall mention first those which follow the action of 

 some continuous process, and afterward those which depend orv 

 the recognition of rhythms. 



The earliest computations of geologic time, as well as the 

 majority of all such computations, have followed the line of the 

 most familiar and fundamental of geological processes. All through 

 the ages the rains, the rivers and the waves have been eating 

 away the land, and the product of their gnawing has been received 

 by the sea and spread out in layers of sediment. These layers 

 have been hardened into rocky strata, and from time to time 

 portions have been upraised and made part of the land. The 

 record they contain makes the chief part of geologic history, 

 and the groups into which they are divided correspond to the 

 ages and periods of that history. In order to make use 

 of these old sediments as measures of time, it is necessary to 

 know either their thickness or their volume, and also the rate at 

 which they were laid down. As the actual process of sedimen- 

 tation is concealed from view, advantage is taken of the fact that 

 the whole quantity deposited in a year is exactly equalled by 

 the whole quantity washed from the land in the same time, and 

 measurements and estimates are made of the amounts brought to 

 the sea by rivers and torn from the cliffs of the shore by waves. 

 After an estimate has been obtained of the total annual sedi- 

 mentation at the present time, it is necessary to assume either 

 that the average rate in past ages has been the same or that it 

 has differed in some definite way. 



At this point the course of procedure divides. The computer 

 may consider the aggregate amount of the sedimentary rocks, 

 irrespective of their subdivisions, or he may consider the thick- 

 nesses of the various groups as exhibited in different localities. 

 If he views the rocks collectively, as a total to be divided by the 

 annual increment, his estimate of the total is founded primarily 

 on direct measurements made at many places on the continents, 

 but to the result of such measurements he must add a f>ostulated' 

 amount for the rocks concealed by the ocean, and another post- 

 ulated amount for the material which has been eroded from the 

 land and deposited in the sea more than once. 



If, on the other hand, he views each group of rocks by itself, 

 and takes account of its thickness at some locality where it is 

 well displayed, he must acquire in some way definite concep- 

 tions of the rates at which its component layers of sand, clay and 

 limy mud were accumulated, or else he must postulate that its 

 average rate of accretion bore some definite ratio to the present 

 average rate of sedimentation for the whole ocean. This course 

 is, on the whole, more difficult than the other, but it has yielded 

 certain preliminary factors in which considerable confidence is 

 felt. Whatever may have been the absolute rate of rock build- 

 ing in each locality, it is believed that a group of strata which 

 exhibits great thickness in many places must represent more 

 time than a group of similar strata which is everywhere thin, 

 and that clays and marls, settling in quiet waters are likely to 

 represent, foot for foot, greater amounts of lime than the coarser 

 sediments gathered by strong currents ; and studying the forma- 

 tions with regard to both thickness and texture, geologists have 

 made out what are called time ratios — series of numbers express- 

 ing the relative lengths of the different ages, periods and epochs. 

 Such estimates of ratios, when made by different persons, are 

 found to vary much less than do the estimates of absolute time, 

 and they will serve an excellent purpose whenever a satisfactory 

 determination shall have been made of the duration of any one 

 period. 



Reade has varied the .sedimentary method by restricting atten- 

 tion to the limestones, which have the peculiarity that their 

 material is carried from the land in solution ; and it is a point 

 in favour of this procedure that the dissolved burdens of rivers 

 are more easily measured than their burdens of clay and sand. 



An independent system of time ratios has been founded on 

 the principle of the evolution of life. Not all formations are 

 equally supplied with fossils, but some of them contain volu- 

 minous records of contemporary life ; and when account is takers 

 of the amount of change from each full record to the next, the 

 steps of the series are found to be unequal in magnitude. Though 

 there is no method of precisely measuring the steps, even in a 

 corriparative way, it has yet been found possible to make 

 approximate estimates, and these in the main lend support to 

 the time ratios founded on sedimentation. They bring aid also- 

 at a point where the sedimentary data are weak, for the earliest 

 formations are hard to classify and mea.sure. It is true that 

 these same formations are almost barren of fossils, but biological, 

 inference does not therefore stop. The oldest known fauna. 



