CHAPTER IV. 



STRATIFIED ROCKS— THEIR NATURE. NOMENCLATURE, 

 AND FOSSIL CONTENTS. 



The Common Usage in Classifying Stratified Rocks. — As de- 

 fined on a previous page, geological systems are the primary 

 units of the time-scale ; they are also the grand divisions 

 made in classifying stratified rocks. When terms indicating 

 lapse of time are applied to these divisions, the meaning is 

 lapse of time during which the system was forming. There 

 is a Carboniferous period only as it is the unknown lapse of 

 time during which certain strata included in a Carboniferous 

 system were forming. The limits of that time are determined 

 only by the unknown points of time when the first and the 

 last strata of the system were laid down. The thickness and 

 kind of rock, or other phenomena, may give us a clue to the 

 possible duration measured between the two points, but it is 

 a mistake to imagine that we know anything of the particular 

 geological time, period, era, or epoch at which a particular 

 stratum was made, except as indicated by the fossils which 

 record the age. The laying down of a particular sandstone 

 at a particular place marked a definite point in time, though 

 we may not know in terms of years, or centuries, or millions 

 of years, how long ago it was, and it is the stratum, and not 

 the period, that is definite. 



Fossils of Higher Value than Strata for Determining Time- 

 relations. — According to general usage the fossils are not sup- 

 posed to be the time-indicators, but the stratum is supposed 

 to be the indicator of the age of the fossil. This common 

 usage is defective, in that fossils, when considered as the re- 

 mains of races of organisms regularly succeeding one another, 

 record the steps of progress made in their evolution and may 



