1879.] of life forms to breaks of continuity in the strata. 249 



fluviatile and lacustrine deposits? How many of the forms of life 

 represented in the old upheaved bed of the Mediterranean on the 

 north could be found in the waters and on the shore of the ocean in 

 which the once midland sea was merged by the folds of earth's 

 crumpling crust? 



But the migration would not be necessarily, or even generally, 

 only to the newly-submerged or newly-raised areas* An unsettling 

 of the life stations in any area would, cause those forms which 

 could migrate to appear suddenly in adjoining areas where no 

 movem-ents were going on, so that their remains would appear 

 in the middle of a continuous series of deposits. And the move- 

 ment might not be from north to south across the equatorial re- 

 gion, so that many forms which could" not endure extremes of heat 

 or cold might travel on for ever round the earth if the movements 

 did not necessitate their crossing unsuitable climes. 



The short sketch I have just given is a fair sample of what has 

 been going on over and over again on various parts of the earth's 

 surface. If then we can read in the rocks the evidence of such 

 succession of events as gav^^^'i many times sea where there had 

 been land, and land where''' tire had been sea, and we can find 

 traces of the successive forilir^ of life, it will be interesting to 

 enquire what is the relation between the appearance upon the 

 earth of distinct forms of life and +be great changes in the physi- 

 cal geography of the areas over which they are found. 



I can only gather a few examplf^s- lere and there, but I think 

 it will be seen that it is a line of enqi*^ii7 for which a vast quantity 

 of evidence is being rapidly accumulat^ec. 



Taking the oldest rocks of which we know anything in Britain, 

 I refer you for a moment to that ancieni series I brought before 

 your notice on a former occasion, when I had just worked out 

 their relations near Bangor and Carnarvon. These are the Pre- 

 Cambrian rocks, perhaps the equivalents of the Huronian of 

 America, but for the purposes of our enquiry to-day I refer to them 

 only to dismiss them as we have not got a trace of life in them. It 

 is true that in America traces of fossils are found in beds probably 

 far older than our Carnarvon and Bangor beds, but these not 

 universally admitted to be of organic origin ; and in Britain we 

 have nothing of the kind. 



These ancient deposits were crumpled, raised above water, went 

 down again, and on the irregular submerged land the Cambrian 

 rocks were laid with here and there a shingle beach, and here and 

 there sand and mud. 



Referring to the table of strata, you will see that I take to- 

 gether under the old name Cambrian, under which they were 

 first described, all the rocks from the conglomerates at the base 

 of the Llanberis and Harlech groups to the top of the Bala Beds. 



18—2 



