INTRUSIVE MASSES 277 



Lava Flows and Sheets which were poured out on the surface 

 of the ground may be recognized by the aid of several criteria. 

 In flows of only moderate antiquity, which have suffered little 

 denudation, the nature of the mass may be determined at a glance, 

 and traced to the vent whence it issued. Successive sheets, piled 

 one over the other in a rude bedding, are also evidence that the 

 rocks are surface lavas. (See Fig. 21, p. 57.) Surface sheets may 

 be overlaid by sediments, deposited upon a submarine flow, or 

 after depression of the land. Such a flow is then called a con- 

 temporaneous or interbedded sheet, and evidently its geological age 

 follows the rule for strata ; it is newer than the bed upon which 

 it lies and older than the one which rests upon it. 



Fragmental Products (Pyroclastic) are positive proof of vol- 

 canic action, for they cannot be formed underground. Coarse 

 masses of agglomerate, blocks, and bombs show that the vent 

 from which they issued was not far away, while beds of fine ashes 

 and tuffs may be made at great distances from their source. All 

 these varieties may be enclosed in true sediments, and may, in part, 

 escape destruction long after the volcano which ejected them has 

 been cut away. The fragmental products are always contempo- 

 raneous, and when interstratified with sediments are newer than 

 the underlying, older than the overlying stratum. 



II. Rocks solidified below the Surface (Plutonic) 



We now come to a series of rocks which no one has ever ob- 

 served in the course of formation, because they were solidified at 

 greater or less depths beneath the ground. When such masses 

 are exposed to view, it is not because they have been brought to 

 the surface, but because the surface has been eroded down to 

 them. Though these unstratified masses cannot be observed in 

 the process of formation, as may the lavas and pyroclastic rocks, 

 yet the nature of the rocks themselves, and their relations to the 

 volcanic and stratified rocks, enable us to explain them satisfac- 

 torily. In whatever shape they occur, these masses are intrusive, 

 and have forced their way upward, filling fissures and cavities, 



