THE MIOCENE PERIOD. 



273 



the lavas, but the extrusions were by no means confined to the areas 

 where Miocene sedimentation had been in progress. 



While igneous activity has been in progress interruptedly since 

 the earliest known times, the record of few periods of geological his- 

 tory shows such extraordinary extrusions of lava as those of the Ter- 

 tiary. The exact stage of the Tertiary at which the great lava sheets 

 of the west were extruded has not been determined in all cases; but 

 the lavas of afr least a considerable part of 200,000 or 300,000 square 

 miles of lava-covered country in the western part of the United States 



Fig. 451. — Sections of petrified logs, near Holbrook, Ariz. Age of beds not known. 



issued during the Miocene period, or during the time of crustal defor- 

 mation which brought it to a close. 



The volcanic activity of the time was not restricted to the Cor- 

 dilleran system, but affected also the Antillean system of Central 

 America and the West Indies, 1 and the Andean system of South 

 America. 



Close of the Miocene. — During the Miocene, there appears to have 

 been more or less crustal movement throughout the Cordilleran region. 

 Slow warpings of the surface seem to have been in progress, while 



1 Hill, Geology of Jamaica. Reviewed in Jour, of Geo!., Vol. VII. 



