358 CAMBRIAN PERIOD 



required a very long period of time; but since there is no standard 

 rate at which any sort of sediment accumulates, 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 accumula- 

 tion, 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, this period would be lengthened greatly. It should be 

 remembered, however, that while one foot per century is a rate 

 at which limestone may accumulate, it does not follow that it is 

 the rate at which Cambrian limestone was formed. 



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. For aught that is now 

 known, the largest of these figures might be multiplied by 2 or 

 even by some larger number. 



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 beginning 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, the oldest accessible Archean formations. If so, it 

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

 known from fossils. The known fossils from the Proterozoic rocks 

 give but a very inadequate conception of life before the Cambrian. 

 But in the Cambrian system there is, for the first time, a reasonably 

 adequate record of animal life. 



Animal fossils. Every great division of the animal kingdom, 

 except the vertebrate, was representated in Cambrian times, and 

 though no vertebrate remains have yet been found, it would be rash 

 to assume that no vertebrates lived. All the known fossils appear 

 to be of marine species. Of land animals there are no traces, but 

 this does not prove that they did not exist. 



1 For a general discussion of this topic, see Williams' Geological Biology, Chap. II. 



