BREAKS IN THE GEOLOGICAL RECORD. 51 



to the causes above enumerated, is greatly aggravated, especi- 

 ally as regards the earlier portion of the earth's history, by the 

 fact that many rocks which contained fossils when deposited 

 have since been rendered barren of organic remains. The 

 principal cause of this common phenomenon is what is known 

 as " metamorphism " that is, the subjection of the rock to a 

 sufficient amount of heat to cause a rearrangement of its par- 

 ticles. When at all of a pronounced character, the result of 

 metamorphic action is invariably the obliteration of any fossils 

 which might have been originally present in the rock. Meta- 

 morphism may affect rocks of^any age, though naturally more 

 prevalent in the older rocks, and to this cause must be set 

 down an irreparable loss of much fossil evidence. The most 

 striking example which is to be found of this is the great Lau- 

 rentian series, which comprises some 30,000 feet of highly- 

 metamorphosed sediments, but which, with one not wholly 

 undisputed exception, has as yet yielded no remains of living 

 beings, though there is strong evidence of the former existence 

 in it of fossils. 



Upon the whole, then, we cannot doubt that the earth's 

 crust, so far as yet deciphered by us, presents us with but a 

 very imperfect record of the past. Whether the known and 

 admitted imperfections of the geological and palaeontological 

 records are sufficiently serious to account satisfactorily for the 

 deficiency of direct evidence recognisable in some modern 

 hypotheses, may be a matter of individual opinion. There 

 can, however, be little doubt that they are sufficiently extensive 

 to throw the balance of evidence decisively in favour of some 

 theory of continuity, as opposed to any theory of intermittent 

 and occasional action. The apparent breaks which divide the 

 great series of the stratified rocks into a number of isolated 

 formations, are not marks of mighty and general convulsions 

 of nature, but are simply indications of the imperfection of 

 our knowledge. Never, in all probability, shall we be able to 

 point to a complete series of deposits, or a complete succession 

 of life linking one great geological period to another. Never- 

 theless, we may well feel sure that such deposits and such an 

 unbroken succession must have existed at one time. We are 

 compelled to believe that nowhere in the long series of the 

 fossiliferous rocks has there been a total break, but that there 

 must have been a complete continuity of life, and a more or 

 less complete continuity of sedimentation, from the Laurentian 

 period to the present day. One generation hands on the 

 lamp of life to the next, and each system of rocks is the direct 

 offspring of those which preceded it in time. Though there 



