4-02 



HISTORICAL GEOLOGY 



The Cambrian Period 



Divisions of the Cambrian. — The first great period of the Paleo- 

 zoic is the Cambrian (Latin name for Wales), so-called because of its 

 development in Wales, where it was first studied with care. This is 

 the oldest fossiliferous system known at present (although a new series 

 of fossils may yet be discovered in the youngest Proterozoic rocks), 

 if one excepts the few fossils found in the Proterozoic ; and upon it we 

 must depend to a large extent for our knowledge of the early life of 

 the world. 



The Cambrian is usually separated into three subdivisions : the 

 Lower (Waucobian), the Middle (Acadian), and the Upper (Croxian). 

 These divisions are based upon differences in the character of the sedi- 

 ments in certain regions, but chiefly upon the differences in the faunas. 

 A study of the fossils of the Cambrian formations has shown (as is 

 true of all later systems) that the fossils of the earliest and latest 

 formations of the system differ markedly, although some of them are 

 the same. This is due to the gradual disappearance of some species 

 and the introduction of others. Among the trilobites in the Lower 

 Cambrian (p. 412) is a world-wide genus (Olenellus, Fig. 382 A, p. 412) 

 which is not found in the Middle Cambrian, while in the Middle Cam- 

 brian a trilobite appears (Paradoxides, Fig. 382 B, p. 412) at about 

 the time that the Olenellus drops out. The Upper Cambrian like- 

 wise is distinguished by the presence of another genus (Dicellocepha- 

 lus, Fig. 382 C, p. 412). The fact that these trilobites are practically 

 restricted to one series each has given rise to the use of their names in 

 indicating the divisions of the system. Thus the life of the Lower 

 Cambrian is spoken of as the Olenellus fauna, that of the Middle 

 Cambrian as the Paradoxides fauna, and that of the Upper Cambrian 

 as the Dicellocephalus fauna. Not only are certain trilobites char- 

 acteristic of these three epochs of the Cambrian, but other forms of 

 life as well, so that even though trilobites are absent, the age of the 

 rocks can be determined by other genera and species. Although 

 certain genera and species are practically confined to one formation, 

 others have a wide vertical range, i.e., are found in several formations. 

 Such fossils, while showing that the rocks are of Cambrian age, do 

 not, without the presence of those of more restricted vertical range, 

 tell to which series they belong. 



Location of Cambrian Rocks. — The Cambrian formations outcrop 

 around the borders of the Pre-Cambrian rocks; as, for example, on 



