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 paleontological 
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 
