UN STRATIFIED OR IGNEOUS ROCKS. 



219 



3. Intercalary Beds. Often sheets are found be- 

 tween the strata, sometimes repeated many times. In 

 such cases they may have been poured out on the bed of 

 the sea or lake, and covered with sediment ; or they may 

 have broken through the strata for a certain distance, 

 and then spread between the separated strata (Fig. 126). 

 Both of these cases occur. If the strata both above and 

 below the sheet are changed by heat, then it has been 

 forced between ; but if only the underlying stratum is 

 changed, then it has been outpoured on the bed of the 

 sea or lake, and covered with sediment. 



Age of Eruptives. Where two dikes or streams 

 meet, their relative ages may be known. In case of suc- 

 cessive streams, that which covers is of course the later. 

 If one dike intersects another (Fig. 127), the intersecting 

 dike, a, is the younger. The absolute age, i. e., the geo- 



FIG. 127. 



logical period when the eruption took place, can be de- 

 termined only by the age of the associated stratified rocks. 

 If igneous rocks break through, or are outpoured upon, 



FIG 



or forced between layers of stratified rocks, then the 

 igneous rock must be younger ; but if intercalary beds 



