1440 GEOLOGY. 



The fossils in the strata of any division of the Cambrian constitute 

 the known fauna of that stage. The fossils found probably do not 

 represent the full fauna of any stage, since it is hardly to be supposed 

 that relics of all species have been preserved. The term fauna has both 

 a geographic and a geologic significance. The Upper Cambrian fauna 

 includes all forms of animal life of the Upper Cambrian epoch, while 

 the Upper Cambrian fauna of New York includes only those forms which 

 lived in that region at that time. 



With the possible exception of the Atlantic coast region, notably 

 Newfoundland, the oldest known part of the Cambrian system contains 

 certain fossils which are distinctive. Among them are the species of a 

 genus of trilobites known as Olenellus (Fig. 118 a, p. 281). Along with 

 the representatives of the genus Olenellus, there are many other species 

 of various types of life. To the aggregate, the name Olenellus fauna * 

 has been given. It is not to be understood that representatives of the 

 genus Olenellus itself are found at all points where the lower part of 

 the Cambrian system is present, or that other genera of trilobites are 

 absent. Wherever Olenelli are found, various species of other genera 

 of trilobites occur, as well as various species of genera of other groups 

 of animals, and wherever these other species of the Olenellus fauna are 

 found, the Olenelli themselves may be expected. Geologists have 

 very generally agreed to regard the base of the formations containing 

 the Olenellus fauna as the base of the Cambrian system; 2 this defi- 

 nition of the base of the system has been found applicable both in 

 America and Europe, and in both continents the Olenellus fauna is 

 the oldest known abundant fauna. Conformable beds which lie 

 below the Olenellus horizon, and whose character gives evidence that 

 they were deposited immediately preceding the Olenellus beds and 

 under the same general conditions, are better classed as Paleozoic 

 than as Proterozoic, and they may be tentatively referred to the Cam- 

 brian. 



As time went on, the inhabitants of the sea changed. Where the 

 Cambrian formations represent a sufficiently long interval of time, 

 the fossils in the lower beds are not altogether like those above. At no 

 single plane is there, as a rule, a very notable change in species, but 



1 For a general discussion of the Olenellus fauna, see Walcott, Tenth Ann. Rept. 

 U. S. Geol. Surv. 



2 Walcott, Bull. 81, U. S. Geol. Surv. 



