334 



PHYSICAL GEOLOGY 



columns usually results, and the direction of the columns is at right 

 angles to the cooling surface. The reason for the horizontal position 



Fig. 330. — Diagrams showing the origin of basaltic jointing. In shrinking, the 

 least number of cracks that will relieve the tension in all directions, A> is three. Similar 

 radiating cracks from other centers complete the six-sided prism, B. When cracks 

 fail to develop about some one point, a five-sided prism, C, results. (Modified after 

 Chamberlin and Salisbury.) 



of the columns of vertical dikes and the vertical position of those 

 of lava flows is thus explained (Fig. 330 A-C). 



Age of Igneous Rocks 



The exact age of ancient volcanoes or of igneous intrusions can 

 seldom be ascertained, but the relative age is often known. The 

 relative age is determined as follows : a volcanic neck is clearly younger 

 than the rocks which it penetrates ; a laccolith or sill is of later age 

 than the beds in which it was intruded, and a lava flow is more recent 

 than the formations over which it spreads. The eruption or intru- 

 sion in each case could not have taken place before these rocks were 

 laid down. If on the other hand pebbles of igneous rock are found 

 in sedimentary rocks, we know that the rocks from which they were 

 derived were at the surface before the sediments were deposited, or 

 while the deposition was taking place. For example, if Devonian 

 strata are cut by a volcanic neck, we know that the neck is younger 

 than the Devonian, and if pebbles from this same neck are found in 

 Middle Carboniferous sediments, it is evident that the lava was prob- 

 ably intruded in early Carboniferous times. Sometimes the presence 

 of fossils In volcanic tuff shows definitely at what time the eruption 

 occurred. 



Theories of Volcanism 



So many theories of volcanism have been offered that it is impossible 

 in an introductory volume to do more than briefly indicate a few of 



