THE CAMBRIAN PERIOD 417 



brian was continuous, but more rapid at certain times than at others, 

 the fauna at the close of the period being distinctly more advanced 

 than that at the beginning. The evolution of life was profoundly 

 influen:ed by environment, this perhaps more than any other cause 

 being responsible for the marked difference between the faunas oi 

 the Lower and Upper Cambrian. 



Since the life of the Cambrian changed from time to time during 

 the period, a study of the fossils of any stratum, as has been said, 

 gives definite information as to the relative age of the beds containing 

 them. This change in the fauna was brought about (1) by the slow 

 evolution of species when conditions were somewhat uniform ; (2) by 

 rapid evolution due to changes in environment, such as occurred 

 when seas were enlarged, shore lines shifted, and new conditions of 

 food and temperature imposed ; (3) by competition resulting from the 

 immigration of large numbers of new species from other regions, which 

 caused the extinction of many species and the modification of others. 



Climate and Duration. — The widespread occurrence of coral-like 

 organisms in the Lower Cambrian and the vast numbers of individuals 

 of various species of trilobites and other classes indicate a warm and 

 more or less uniform climate. In fact, throughout at least the greater 

 part of the period the character and distribution of the fossils imply 

 nearly uniform climatic conditions over the entire world. 



The duration of the Cambrian is to be expressed in terms of hun- 

 dreds of thousands of years. The time required to remove and de- 

 posit thousands of feet of rock must have been enormous. If lime- 

 stone is deposited on an average of one foot a century, it would re- 

 quire 600,000 years for the accumulation of the 6000 feet of Cambrian 

 limestone of some portions of the West, omitting the time necessary 

 for the formation of the thick sandstones of the same regions. Per- 

 haps 1,000,000 years may be placed as the minimum duration of the 

 period and 4,000,000 years as the maximum. 



REFERENCES FOR THE CAMBRIAN PERIOD 

 Walcott, C. D., — The Cambrian Faunas of North America: Bull. U. S. Geol. Surv. 

 No. 10, 1885. , M'AM O 



Walcott, C. D., — Studies in the Cambrian Faunas oi * * , , • A U. S. 



„ ' ' .ely submerged since th 



Geol. Surv. No. 30, 1886. y 



Walcott, C. D., — Fauna of the Lower Cambrian. &• sUll . r .., »,.. ... Geoi. 



Surv., 1890, pp. 515-629. 

 Walcott, C. D., — Cambrian Brachiopoda: Mon. 51. U. S. Geol. Surv., 1912. 

 Walcott, C. D., — Cambrian Geology and Paleontology: Smithsonian Misc. Coll. 



Vol. 53, 1908, pp. 1-230; and Vol. 57, 1910, pp. 1-16, 231-431. 



