DISTRIBUTION OF FOSSILS IN THE STRATA. 195 



water or brackish-water deposits of sand and clay. It is, therefore, in 

 such strata that their remains are commonly found. 



2. The Country where found. — It is also well known that the 

 faunas and floras of different countries at the present time differ as 

 to species, and often as to genera and families ; the difference being 

 generally in proportion to the difference in climate, the physical bar- 

 riers intervening, and the length of time during which the barriers 

 have existed. The same was true of the faunas and floras of previous 

 epochs, and therefore of the fossils of the same age in different coun- 

 tries. The fossil species of the same epoch in America, in Europe 

 and in Asia, are not usually identical, although there may be a general 

 resemblance. The geographical diversity, however, is small in the 

 lowest and oldest rocks, and becomes greater and greater as we pass 

 upward into newer and newer rocks, and is greatest in the fauna and 

 flora of the present day. 



3. The Age. — This introduces the subject of the laws of distri- 

 bution of organisms in time, or of fossils vertically in the series of 

 stratified rocks. The subject will be more fully treated in Part III, 

 of which it constitutes the principal portion. We now bring out only 

 so much as is necessary as a basis of classification of stratified rocks. 



(a.) Geological Fauna and Flora. — As we pass from the oldest and 

 lowest rocks upward to the newest and highest, we find that all the 

 species, most of the genera, and many of the families, change many times. 

 Now, all the species of animals and plants inhabiting the earth at one 

 time constitute the fauna and flora of that geological time. Geological 

 faunas, therefore, have changed many times. In a conformable series 

 of rocks the change from one fossil fauna or flora to another succeeding 

 is always gradual, the species of the later fauna or flora gradually 

 replacing those of the earlier. But between two series of unconform- 

 able strata the change is sudden and complete— as if one fauna and 

 flora had been suddenly destroyed and another introduced. It must be 

 remembered, however, that unconformity always indicates a great lapse 

 of time unrepresented at the place of observation by strata or fossils. 

 It is therefore probable that the apparent suddenness of the change is 

 only the result of our ignorance of the fauna and flora of the period un- 

 represented. Nevertheless, as unconformity always indicates changes 

 of physical geography, and therefore of climate, it is probable that in 

 the history of the earth there were periods of great changes, marked 

 by unconformity of strata, during which changes of species were more 

 rapid, separated by periods of comparative quiet, marked by conformity, 

 during which the species were either unchanged, or changed slotoly. 

 Such a period is called a geological period or geological epoch, and 

 the rocks formed during a geological period, or epoch, is called a 

 formation. 



