96 GEOLOGY. 



and the basis of a new fauna is being laid by severe natural selection. 

 But ideally, the expansions and restrictions of the land life are pre- 

 cisely reciprocal to those; of sea life, and hence the centers of these 

 normal land periods, are coincident with the dividing points of the 

 marine periods, as illustrated in Fig. 370. 



When the land period is very pronounced, as after a great deform- 

 ative movement, it is apt to be seriously affected by topographic and 

 climatic agencies, and may not be truly expansional in its life evolu- 

 tion, although it may be revolutionary. Such an instance is the 

 Permo-Triassic land period, when aridity and glaciation probably 

 more than offset the increase of land-area in their influence on organic 

 productiveness. But when the deformative movement did not reach 

 such lengths, and a favorable climate and topography accompanied 

 an increase of land -area, there should naturally be an expansional 

 evolution of the land life. At such times also, the mild deformations 



Fig. 370. — A sketch illustrating the reciprocal relations of ideal land periods and 



sea periods. 



should have developed shallow lodgment -basins, and areas of aggra- 

 dation favorable for a good record of the land life. These theoretical 

 sequences seem to have been realized in the transition from the Jurassic 

 to the Comanchean or Lower Cretaceous. The Purbeckian, usually 

 regarded as the closing stage of the European Jurassic, and the Wealden, 

 usually regarded as the opening stage of the Lower Cretaceous of 

 Europe, though they bridge the dividing line of the marine periods, 

 really constitute together the heart of an important period of terrestrial 

 life development. On the American continent the Como, Trinity, and 

 Lower Potomac horizons stand in the same relations. From this stage 

 dates, as we shall see, the initial deployment of the angiosperms, one 

 of the most important vegetal revolutions in geologic history. In 

 this stage also there was a very marked deployment of the great reptiles. 

 It is inconsistent with a normal treatment of reptilian deployment 

 to dissever it along the lines of division that are most appropriate 

 to the marine life, natural as that is in its own field, and best as a gen- 



