THE CRETACEOUS PERIOD 



429 



and other western states, as well as in South America. These 

 were, however, only the first of a series of volcanoes which 

 grew up during the Tertiary period (Fig. 452). The earlier 

 ones became ex- 



,,'' . --V 



tinct so long ago 

 that they have been 

 worn down, but the 

 latest of them we 

 still see in such 

 great peaks as Mt. 

 Rainier and Mt. 



FIG. 452. Section of a Tertiary volcanic cone the 

 upper part of which has been removed by erosion. 

 The lava flows are vertically shaded. They are 

 interbedded with fragmental deposits. 



Shasta. 



Rapid changes in the animal life. We naturally expect 

 to find that the exclusion of the shallow seas which over- 

 lapped the continent in the Cretaceous period, the growth 

 of mountain ranges where there had been lowlands before, 

 and the accompanying changes of climate had a marked 

 effect upon the living things. Thus at the close of the 

 Cretaceous period, the ammonites, which had long been 

 abundant in the seas, died out in a comparatively short time, 

 leaving no descendants. Among the reptiles the change was 

 quite as marked, although not as complete. Almost all the 

 great reptiles which were so characteristic of the Mesozoic 

 era became extinct, and only the smaller forms which we 

 have to-day, such as the snakes, lizards, and turtles, lived 

 on. The crocodiles seem to be the only remaining represen- 

 tatives of the large Mesozoic reptiles. 



At the same time the mammals began a rise which in the 

 next period became extraordinarily rapid. The appearance 

 of the higher mammals in North America seems to have been 

 sudden, as if they had immigrated from some other locality 

 in which they had slowly developed from simpler forms. If 

 this is true, the place of their origin is not yet known. Whether 

 or not the rapid spread and growth of the mammals was 

 responsible for the disappearance of the great reptiles is an 

 open question, but the suggestion is at least plausible. 



B. & B. GKOL. 



25 



