494 GEOLOGY 



accumulate, this long period cannot be reduced to years. It has 

 been estimated that limestone sometimes forms at some such 

 rate as one foot per century. In some parts of the West there are 

 6,000 feet of limestone, besides thick bodies of fragmental rock. 

 At the above rate of accumulation, the 6,000 feet of limestone 

 would call for a period of 600,000 years, and if time be allowed 

 for the other formations of the same region, the period would be 

 greatly lengthened. It should be remembered, however, that 

 while one foot per century may be a rate at which limestone some- 

 times accumulates, it does not follow that it is the rate at which 

 the Cambrian limestones were formed. The data on which this 

 estimated rate is based are believed to give too high, rather than 

 too low a rate, and a less rapid accumulation would mean a cor- 

 respondingly longer period of time. 



Many estimates of geological time, based on various data, have 

 been attempted. 1 These estimates, so far as applied to the Cam- 

 brian, generally assign to that period a duration of 1,000,000 to 

 3,000,000 years. It should be distinctly borne in mind, however, 

 that the chief value of these figures is to give emphasis to the fact 

 that the period was one of great duration. 



THE LIFE OF THE CAMBRIAN 



Perhaps no single event in the history of the earth possesses 

 greater interest than the first appearance of life; but the date of 

 its advent is not known. There is good evidence that life existed 

 before the close of the Archeozoic era, and under the accretion 

 hypothesis, it is not improbable that its beginning antedated, by a 

 long period, even the oldest accessible Archean formations. If so, 

 it is quite beyond hope that the earliest forms of life will ever be 

 known from its relics. The evidences of plants and animals in the 

 Proterozoic era are so indirect, obscure, or meager that they give 

 but a very inadequate conception of the life before the Cambrian 

 period. The information which the imperfect fossils give, indicates 

 that they do not tell even the main part of the story of Proterozoic 



1 For a general discussion of this matter, see Williams' Geological Biology, 

 Chap. II. 



