50 INTRODUCTION. 



complete continuity of deposition, from the Laurentian period 

 to the present day. There was, and could have been, no 

 such continuity 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 un- 

 broken. 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 palseontological breaks, or by both combined. 

 The utmost we can at present do is to arrive at the con- 

 viction 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 prac- 

 tice 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 : 1. The Atlantic ooze or " abyssal 

 mud " is a whitish or greyish-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 especially 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 composed of the microscopic shells of 



