212 



UNSTRATIFIED OR IGNEOUS ROCKS. 



Fig. 186.— Laccolite (after Gilbert). 



neath. It seems more prob- 

 able, however, that it is 

 rather a question of liquid- 

 ity than of specific gravi- 

 ty. If the liquidity is per- 

 fect as in basalts, then it 

 comes to the surface and 

 outpours, and may extend 

 to very great distances ; 

 but, if, on the contrary 

 the lava is only a stiffly viscous, semi-fused mass, like trachyte and, 

 rhyolite, it may lift up the strata on its back in a dome. 



Age— how determined. — When two dikes intersect each other, then, 

 of course, the intersecting must be younger than the intersected dike. 

 In this manner the relative age of dikes intersecting the same region 

 may often be determined. The absolute age of igneous rocks can only 

 be determined by means of the strata with which they are associated. 

 If a dike is found intersecting strata of known age (&, Fig. 180), the 

 dike must be younger than the strata. If a dike (&'), intersecting 

 strata and outcropping on the surface, is found overlaid by other strata 

 through which it does not break, then the igneous injection is younger 

 than the former and older than the latter. The series of events indi- 

 cated is briefly as follows : first, the older series of sediments has been 

 formed ; then fissures formed and filled by igneous injection ; then 

 erosion has carried away the upper portion of the strata and its in- 

 cluded dike, so that the dike outcrops along the eroded surface; and, 

 lastly, the whole has been submerged and again covered with sediment. 

 In the case of intercalary beds of igneous rocks, if the strata above 

 and below are both metamorphosed by heat, then the fused matter has 

 been forced between and is younger than the strata ; if, however, the 

 underlying stratum is changed but the overlying is not, then the igne- 

 ous matter has been outpoured on the sea-bed and covered with sedi- 

 ment, and is, therefore, of the same age as the strata. The same prin- 

 ciples determine the age of sheets and streams. If sheets are successively 

 outpoured, one atop the other, then, of course, the order of superposi- 

 tion determines their relative age. So, also, if two streams run across 

 each other, the overlying is the younger. In this way Richthofen and 

 others have determined the order of succession of different kinds of 

 tertiary eruptives. Absolute age, or the geological time of eruption, 

 can only be determined by the age of the associated strata. 



Of Certain Structures found in many Eruptive Rocks. 



Columnar Structure. — Many kinds of eruptive rock exhibit some- 

 times a remarkable columnar structure. This is most conspicuous in 



