424 THE CAMBRIAN. [bull. 81. 



It is only by accident and through peculiarly favorable conditions that 

 land faunas have been preserved. With the marine faunas on the con- 

 trary the opportunities for the preservation of the fauna of each epoch 

 were far greater, and the conditions of environment were more favorable 

 for the uniform advancement and evolution of life over extended areas. 

 Life-zones. — The proof of the presence of life-zones in the geologic 

 series is one of great importance in correlation. The subject has been 

 so ably presented by Prof. H. A. Nicholson that I will quote it in full 

 in preference to giving a synopsis of it or to presenting my own views, 

 which accord essentially with those of the author. He says : 



While each geological rock-system is characterized by a general assemblage of dis- 

 tinctive types of animals and plants, the minor subdivisions of each system are like- 

 wise distinguished by the prevalence of particular forms of life. There are no doubt 

 cases in which an extensive series of successive strata may appear to be characterized 

 throughout by essentially the same organic types, there beiug apparently no restric- 

 tion of special fossils to special horizons in the series. In so far as such cases have 

 any real existence, they may be explained as instances in which a great series of sedi- 

 ments has been accumulated with such rapidity that there has been no time for 

 marked biological changes, resulting in the dying out of old species and the intro- 

 duction of new forms. In many cases, however, the apparent diffusion of the same 

 kinds of fossils from the base to the summit of a series of beds perhaps two or three 

 thousand feet in thickness is due simply to the fact that the organic remains met 

 with in the formation have not been sufficiently investigated and that the exact 

 horizon at which each occurs in the series has not been accurately determined. The 

 determination of the horizons of particular life-forms is a work of time and demands 

 both great stratigraphical knowledge and also a wide and accurate acquaintance 

 with the characters of the fossils themselves, two requirements rarely fulfilled in the 

 same individual. 



In a considerable number of cases, however, it has now been shown that the fossils 

 of a given formation may be divided into two principal groups. In the one group is 

 comprised a series of common forms of life, which may be regarded as characterizing 

 the formation as a whole. In the other group are included certain special fossils con- 

 fined to particular parts of the formation and characteristic of certain definite hori- 

 zons or zones, within the limits of the formation. All the great formations are to some 

 extent capable of being broken up into minor rock-groups, characterized by special 

 life-forms. Some of the differences in the kinds of fossils found in different parts of 

 the same formation must, of course, be simply set down to the fact that different 

 kinds of sediment imply changed conditions in the sea, and hence changes in the 

 marine fauna. If, for example, part of a formation consisted of limestone and part of 

 sandstone, we should expect, beforehand, to find that each of these rock-groups would 

 have some fossils not found in the other, since the two would have been formed under 

 different conditions. Apart, however, from differences arising from causes of this 

 nature, we meet with cases in which a formation, even if essentially homogeneous in 

 its mineral nature, can be divided into zones, each of which is characterized by the 

 possession of special groups of fossils. Organisms belonging to any class of animals 

 may serve in this way as test forms (" Leit-Fossilien ") for special horizons in a series 

 of stratified formations, but there are paiticular groups of fossils which have been 

 found to be preeminently available for this purpose. Among the older rocks of the 

 earth's crust, the forms which have proved specially valuable for the determination 

 of particular " zones" are the Graptolites, the Trilobites, and the Brachiopods, while 

 the Cephalopods have been found to afford the most satisfactory tests in the case of 

 the Secondary rocks. A well known instance of this subdivision of a system of strata 

 by means of special types of fossils is that afforded by the Ordovician and Silurian 



