PALAEONTOLOGY 129 



Epigene or superficial agencies 

 Air 



„ T , / terrestrial 

 Water S 



| oceanic 

 Erosion and sedimentation 

 Animal and plant life. 



PALAEONTOLOGY 



In studying an extensive series of geologic formations from 

 bottom to top, we find that through geologic time there has been 

 a progressive advance in the development of animal and plant 

 life as well as a change of genera and species. Forms that are 

 abundant at one horizon seem to have ultimately given up the 

 battle for existence and disappeared, their place in nature being 

 filled by others. So by careful comparison of the animal and 

 vegetable remains found in the different systems and groups and 

 in the minor sub-divisions of the groups, we come to regard the 

 fossils as labels by which we may know the age of strata. While 

 there are some persistent types which pass from one system to 

 another without material change, we find that the life character- 

 istics of each group are essentially distinct. It is therefore im- 

 portant for the field geologist who is studying the formations 

 above the Archaean to be familiar with their fossils in order to 

 determine the horizons accurately. 



In the older formations, plants were few and elementary and, 

 containing but little mineral matter, have not been well preserved 

 so that we depend more on fossil animals than plants for the 

 identification of the Palaeozoic strata. From the Mesozoic on, 

 impressions of land plants are more abundant and become of 

 much value in palaeontology. 



As shown by the fossil remains discovered in rocks of different 

 ages, the development of animal life has been a gradual one, but 

 we are not yet acquainted with any formation which contains the 

 earliest forms of life. We begin our study, as it were, at a some- 

 what late period of life development, the Cambrian, for the fossils 

 of the pre-Cambrian rocks are not yet well known. Somewhere 



