24 INTRODUCTION. 



of these minute organisms. 3. The ooze contains siliceous 

 sponges, in many respects comparable to the sponges which 

 are so characteristic of the Cretaceous period. 4. The ooze 

 contains Echinoderms, especially Sea-urchins and Crinoids, 

 such as abounded in the Chalk period ; whilst one of the latter 

 is related to a Cretaceous type hitherto believed to be extinct. 

 5. We have reason to believe that the conditions under which 

 the Chalk was formed were very similar to those now present 

 in the Atlantic at great depths. 



On the other hand, as pointed out by Sir Charles Lyell and 

 Mr Prestwich, the differences between the Atlantic ooze and 

 and the Chalk are, to say the least of it, quite as weighty as 

 the resemblances, if not more so. Chalk is composed of from 

 eighty to as much as ninety-nine per cent of carbonate of lime, 

 and has therefore a very small proportion of any siliceous 

 or aluminous impurity. Secondly, the occurrence of identical 

 species of Foraminifera in the two formations amounts to very 

 little ; for it is well known that such lowly organised forms of 

 life have an extraordinary power of persistence, surviving geo- 

 logical changes which are fatal to higher organisms. Lastly, 

 the most characteristic of the Chalk fossils, such as the various 

 forms of Cephalopoda and Bivalve Molluscs, are entirely want- 

 ing in the Atlantic ooze. 



Mr Prestwich concludes that although it is probably true 

 that " some considerable portion of the deep sea-bed of the 

 mid-Atlantic has continued submerged since the period of 

 our Chalk, and although the more adaptable forms of life 

 may have been transmitted in unbroken succession through 

 this channel, the immigration of other and more recent faunas 

 may have so modified the old population that the original 

 Chalk element is of no more importance than is the original 

 British element in our own English people. As well might it 

 have been said in the last century that we were living in the 

 period of the early Britons, because their descendants and 

 language still lingered in Cornwall, as that we are living in the 

 Cretaceous period, because a few Cretaceous forms still linger 

 in the deep Atlantic. Period in Geology must not be con- 

 founded with ' system ' or * formation.' The one is only 

 relative, the other definite. A formation is deposited or takes 

 place during a certain time, and that time is the period of the 

 formation ; but a geological period may include several forma- 

 tions, and is defined by the preponderance of certain orders, 

 families, or genera, according to the extent of the period 

 spoken of; and the passage of some of the forms into the next 

 geological series does not carry the period with them, any more 



