The Siskiyou Island. 39 



The newer life of plant and animal adds 

 fresh material to the shore line deposit, and 

 as a result the smaller particles of the rock 

 show increasingly abundant other life than 

 that of the coarser shells. Sheltered bays lent 

 their protection to fragile forms of life, and 

 these so multiplied that the whole mass of 

 the rock becomes eloquent in its record, for 

 then, as now, marine life most abounded in 

 such sheltered places of the beach. 



The chambered shells, represented by the 

 Nautilus and the Ammonite, are found widely 

 distributed in the rocks of the Siskiyou island. 

 The Nautilus survived all the changes of the 

 period and passed on to later times along the 

 newly formed western coast, while its cousin, 

 the Ammonite, of more complex internal 

 structure, became extinct with the close of 

 the Cretaceous. Three of this Ammonite 

 family from Siskiyou island are shown on 

 Plate IV. The causes which brought about 

 its extinction seem to have been connected 

 with the complexity of its structure rather 

 than with its environment, for it drops out of 



