INTRODUCTION 23 



periods, while others are confined to a very small portion of 

 one ; that is, some have a great and others a limited vertical 

 distribution or time endurance. Some are confined to the 

 strata of a single locality, while others, such as the brachiopod, 

 Produdus semireticulatus, occur in strata of similar age over 

 the world; that is, some have a wide and others a limited 

 horizontal distribution or geographic range. The best index 

 fossils are those which combine a wide horizontal with a limited 

 vertical distribution. 



The study of Index Fossils is thus of great importance in 

 determining the age of strata, for the making of geological maps 

 and sections, working out faults, etc. 



Migration, etc. — ■ Fossils tell us of the body of water in 

 which the rocks inclosing them were laid down, — whether 

 shallow or deep, near to or far from land, open sea or inclosed 

 basin. They tell us of changes in climate. They tell us of the 

 presence of land barriers where now are none, of land bridges 

 that formerly united lands now long separated. This knowledge 

 of ancient geography that they give us is based upon the 

 observed fact that species migrate. The larvae of beach- 

 dwelling creatures are carried by tide and waves along the edge 

 of the sea and thus colonize new areas. Large bodies of water, 

 however, few animals can cross, and hence, when the same 

 species of the trilobite Trinucleus, for example, is found in 

 Ordovician deposits of both Europe and America, we must 

 assume a continuous beach line, a land bridge between the two 

 continents at that time. Mammals migrate by walking and 

 swimming, but they, too, are limited in range by broad areas of 

 water, high mountain ranges, etc. Thus the same species 

 of elephant in the Pleistocene strata of both Europe and America 

 indicates a land bridge at that time. 



An example of the rate of speed at which a species may 

 migrate is found in the record of Littorina littorea, the common 

 periwinkle. It is very abundant on the rocky shores of Eng- 

 land. Since it does not occur in. the fossil state, or in any of 



