CONTEMPORANEITY OF STRATA. 23 



as memorials of, at any rate, part of the time which elapsed 

 between the close of the one formation and the commence- 

 ment of the next. Upon any theory of " evolution," at any 

 rate, it is certain that there can be no total break in the great 

 series of the stratified deposits, but that there must have been 

 a complete continuity of life, and a more or less complete 

 continuity of deposition, from the Laurentian period to the 

 present day. There was, and could have been, no such con- 

 tinuity in any one given area ; but the chain could never have 

 been snapped at one point and taken up at a wholly different 

 one. The links must have been forged in different places, 

 but the chain, nevertheless, remained unbroken. From this 

 point of view, there would be little impropriety in saying that 

 we are living in the Silurian period ; but we could only say so 

 in a very limited sense. While most geologists will readily 

 admit that there must have been such an actual continuity of 

 the great geological periods, from the earliest times up to the 

 present day, it remains certain that we can never dispense with 

 the division of the stratified series into definite rock-groups 

 and life-periods. We can never hope to discover all the lost 

 links of the geological chain, and the great formations will 

 always be separated from one another by more or less evident 

 physical or palaeontological breaks, or by both combined. The 

 utmost we can at present do is to arrive at the conviction that 

 the lines of demarcation between the great formations only 

 mark gaps in our knowledge, and that there can be in nature 

 no hiatus in the long series of fossiliferous deposits. 



The theory of " geological continuity," then, may in practice 

 be carried so far as to be useless, or even injurious to the progress 

 of science. This would seem to be the case with the attempt 

 to show that we " are still living in the Cretaceous period," 

 and that the ooze now forming at the bottom of the deep 

 Atlantic is merely a continuation in point of time of the great 

 and well-known formation of the White Chalk. The points of 

 resemblance by which this is sought to be established are 

 these: i. The Atlantic ooze or "abyssal mud" is a whitish 

 or grayish-looking mud, containing about sixty per cent of 

 carbonate of lime, with from twenty to thirty per cent of silica, 

 and a variable quantity of alumina. When dry, and espe- 

 cially if consolidated, it may fairly be compared in mineral 

 composition to some varieties of Chalk or to Chalk-marl. 2. 

 The abyssal mud of the Atlantic is to a very large extent com- 

 posed of the microscopic shells of Foraminifera, some of which 

 are specifically identical with Cretaceous forms, whilst White 

 Chalk is known to be very largely composed of the debris 



