June 1, 1891.] 



KNOWLEDGE 



103 



uess of Sirian over Solar stars. Tlien again, the complete 

 ^'radation observed, not a shade of difference missing, from 

 Sirius down to stars like Mira Ceti, or cc Herculis, is a 

 strong argument in favour of an historic as well as a 

 chemical interpretation of spectral peculiarities. But I 

 would urge that this supposed course of evolution need not 

 be universall}' operative, it is not necessary that every star 

 pass through every stage. It is, perhaps, not necessary 

 that any one star passes through all. lUit just as we may 

 suppose that in a typical star, successive conditions of 

 temperature and condensation bring up into the "reversing 

 layer " at one epoch hydrogen, at another magnesium, 

 at a third the substances, whatsoever they be, that cause 

 the beautiful bands which Mira shows ; so in the wider 

 evolution of a multiple system, or wider still, of the entire 

 Clalaxy, certain stars, certain districts may become richer 

 than the rest in this or that element, to be poorer in 

 others ; so that they have, as it were, a special stage 

 indelibly imprinted on them. Thus they may present the 

 appearance of a given epoch without ever having passed 

 through the one theoretically considered as preceding it, 

 and witliout the possibility of their ever entering on the 

 [ihase sui)posed to be the next. 



To sum up. The Types of Btellar Spectra arc not to 

 be explained along one line only, but along two ; they do 

 not always denote the phases attained by the stars in their 

 development ; they often indicate instead, radical differ- 

 ences of constitution. LSut when they have the first, 

 the historical si,i,aiificauco, the Solar usually denotes a 

 later epoch than the Sirian ; possible exceptions being seen 

 in the cases of y Leouis and Arcturus. And on the average 

 the Solar stars and not the Sirian are the brightest, and 

 have the greatest masses, though they are inferior to the 

 latter as to relative brightness per unit of mass. 



THE ORIGIN OF THE CHALK. 



Uy John T. Kkmi', M. A. Cantab. 



THE origin of the Chalk is a question which, though 

 it has been long under discussion, is not by any 

 means satisfactorily settled. The early investi- 

 gators were much puzzled through being unable 

 to find any rock at all resembling it in process of 

 formation at the present time. Its extreme freedom from 

 clayey or sandy material led them to regard it as a deep- 

 sea deposit ; and, hence, upon the discovery of the Grey 

 Ooze of the Atlantic Ocean, it was concluded, somewhat 

 hastily perhaps, that it must be the missing analogue. 

 Some writers even went so far as to maintain that we were 

 still living' in the Chalk age. The argument was supported 

 not only by tlie occurrence of similar species of foraminifera 

 and other lowly organisms in the two deposits, but by 

 certain resemblances in their higlier fauna. Sponges 

 which abound in tlie Ooze are closely allied to the siplio- 

 nias and ventriculites of tlie Chalk. Echinodei-ms arc 

 conunon in botli. l\c/iiiiothiiiiii, a genus characterized by 

 the possession of a llexible covering, and peculiar to the 

 Cluilk, hnds its nearest living representatives in the deep- 

 water forms, Axtlicniisdwii and I'liiiniiiisdiini. A species of 

 /;«/i/.r, a well-known penus of fishes from the Chalk, has 

 also been found in tlio Ooze. 



That the Ciialk was not formed under precisely tlie same 

 conditions as the Atlantic Ooze appears now to be (juite 

 certain, but whether the dilfereneos are sulKcient entirely 

 to upset the older theory is still a matter of dispute. 



One of these dilVerenccs is in the chemical composition 

 of the two deposits. Whereas the Chalk is composed of 



*98 or 99 per cent, of carbonate of lime with a little silica 

 and alumina, the Ooze only contains from 44 to 80 

 per cent, of carbonate of lime, with .5 to 8 per cent, of 

 silica, and from H to 88 per cent, of alumina. In these 

 analyses it niust, however, be remembered that in the Ooze 

 every constituent is taken into account, while in the Chalk 

 no notice is taken of the Hints which contain the greater 

 part of the silica. Prof. Prestwich estimates them at 

 from 4 to (i per cent, of the whole mass. The dill'crenco 

 in the average amount of silica, if any, is, however, so 

 slight that, taken by itself, it does not militate against the 

 similarity as to ori,L,'in of the two deposits. 



The high proportion of alumina in the Ooze, as compared 

 with the Chalk, presents at first sight a greater difficulty to 

 the acceptance of the older theory. Its abundance in the 

 former deposit is undoubtedly due to mechanical causes, 

 for only the slightest traces of it are present in solution in 

 sea-water, and it is never secreted by living organisms, as 

 silica and carbonate of lime are. This difference in com- 

 position is nevertheless compatible with close similarity of 

 origin. For, supposing that other appearances all pointed 

 in that direction, this one exception might easily be ex- 

 plained on several hypotheses, such as the subsidence of 

 suspended material from tlio land before reaching the Chalk 

 area owing to great distance or slowness of currents ; or 

 secular variations in the amount of meteoric, volcanic, or 

 other kinds of dust falling upon the sea. 



It is when we come to exainiue the stratigraphy of th ^ 

 Chalk more carefully, and to compare its fauna more closely 

 with the fauna of the Atlantic Ooze, that we find the most 

 c5geut reasons for doubting their likeness of origin. 



The White Chalk, as distinguished from other Cretaceous 

 deposits of the same age, is confined to an area stretch- 

 ing from Britain and Northern France throui,'h Holland, 

 Northern Germany, Southern Sweden, and Southern Russia 

 to the Crimea and the Ural Mountains. On the older 

 rocks around this region beach-lines have been discovei-ed 

 at various levels, while throughout the Chalk eroded sur- 

 faces are frequently met with, accompanied by changes of 

 life. One of these breaks, or unconformities, occurs at the 

 base of the zone I of Micinstcr aii-hsiwUwiiiutii, another 

 separates that zone from the zone of MiciasUr cor- 

 iDiiliiinuiii, while a third precedes the incoming of the 

 Danian stage. There are also various otliers. Tlie near- 

 ness of the coasts and the frequency with which the sea- 

 bed was brought within the sphere of denuding action by 

 earth movements are strongly indicative of shallow water. 



The palffontological evidence on the whole strongly 

 confirms this view, notwithstanding, as has already been 

 stated, some apparently adverse facts. 



The late Dr. Gwyn Jeffries finds that the seventy-one 

 genera of Mollusca known in the Chalk are all forms 

 whose closest existing allies inhabit comparatively shallow 

 water, while the genera characterizing the Ooze are very 

 rare or altogether wanting in the older formation. 

 Brachiopoda and Cephalopoda, so abundant in the Chalk, 

 have been shown by the "Challenger" dredgiugs to be 

 very uncommon in the t)oze. Although Echinodermata 

 arc frequent, they are (like the Mollusca) mostly of small 

 size, and but distantly related to the Cretaceous forms. 



Tlie Atlantic Ooze is evidently of exceedingly slow 

 growtli, while the Chalk, on the contrary, appears, from the 



* Tills ii|i|ilio» 111 tin- puri' whili- t'lialk oiih ; thi- (.'lialk miH roii- 

 fciiins a much larp-r iiiTivatago of otIuT iimttfi-. 



tTlic.so "/.oiu's" ari- vrrtical sub-ilivisions of tlicChalk i-liai-.u-k-rizcd 

 by tin- piv.si-uiT uf paHirular .spcfii-s of fossils whicli aiv i-arely, if 

 I'viT, fnimil ah liinlu-r or lowi-r lovfls. Mivmster is a g.-iius of si>a- 

 un-liiii-. 



