196 STRATIFIED OR SEDIMENTARY ROCKS. 



There are, therefore, two tests of a formation and a corresponding 

 geological period, viz., 1. Conformity of the strata or rock-system, and, 

 2. General similarity of fossils, or life-system. There are also two 

 modes of separating formations and corresponding periods, viz., un- 

 conformity of the rock-system, and great and sudden change of the 

 life-system. A geological formation, therefore, may be defined as a 

 group of conformable rocks containing similar fossils, usually separated 

 from other similar groups containing different fossils by unconformity. 

 A geological period may be defined as a period of comparative quiet, 

 during which the physical geography, climate, and fauna, and flora 

 were substantially the same, usually separated from other similar periods 

 by changes of physical geography and climate, which resulted in changes 

 of fauna and flora. Of these two tests, however, the life-system is 

 usually considered the most important, and in case of disagreement 

 must control classification. 



(b.) Geological Faunas and Floras differ more than Geographical 

 Faunas and Floras. — If there were no geographical diversity, species 

 of the same age would be identical all over the earth, and therefore it 

 would be easy to determine strata of the same age (geological horizon). 

 On the other hand, if geographical diversity in any age were as great 

 as the diversity between two successive ages, then it would seem im- 

 possible to establish a geological horizon. But this law states that the 

 difference between two successive faunas is greater than between two 

 contiguous faunas. In other words, the species of successive periods, 

 or fossils of successive formations, differ from each other more than 

 species of the same period or fossils of the same formation in different 

 parts of the earth. There is a general similarity in the species of the 

 same period all over the surface of the earth. Hence by comparison of 

 fossils it is possible to determine what strata, in different portions of 

 the earth, belong to the same period (to synchronize strata). The 

 strata all over the earth, which were formed at the same time, are said 

 to belong to the same geological horizon. Strata of the same horizon 

 are determinable by similarity of fossils with considerable certainty, 

 until we come up to the tertiary rocks. In all the newer rocks, how- 

 ever, the geographical diversity is so great as to interfere seriously with 

 the ability to synchronize by means of comparison of fossils. Another 

 method, therefore, is used for these higher rocks. 



(c.) Increasing Likeness to Existing Forms. — By examining and 

 comparing fossils from the lowest to the highest rocks, it has been ob- 

 served that there is a steady approach of the fossil faunas and floras to 

 the present faunas and floras, first in the families, then in the genera, 

 and finally in the species. The species of fossil molluscous shells begin 

 to be identical with molluscous species of the present day only in the 

 tertiary rocks, and the proportion of identical species steadily increases 



