500 



GEOLOGY. 



affected by columnar structure. In this case, as in all others, the columns 

 are likely to be at right angles to the cooHng surface. Lava solidifying in 



Fig. 384. Fig. 385. 



Fig. 384. — The completion of the hexagonal columns. 

 Fig. 385. — Diagram to illustrate the development of five-sided columns. 



the passageway leading from the interior of a volcano gives rise to a 

 neck or plug. If the lava is forced between beds of rock in the form 

 of a sheet, and solidifies there, it is called a sill. If, after rising to a cer- 

 tain point in the strata, the lava arches the beds above into a dome, and 

 forms a great lens-hke or cistern-hke mass, it constitutes a laccolith (Fig. 

 334). If an intrusion of the laccolithic type faults the overlying beds 

 instead of arching them, and especially if the vertical dimension of the 

 intruded mass be great in comparison with its lateral dimensions, its 

 shape is more like that of a plug or core. Such an intruded core is a 

 hysmalith^ (Fig. 124). Between the bysmalith and the laccolith there 

 are various gradations, just as between the laccolith and the sill. 

 When lava forces aside the rocks at considerable depths or absorbs 

 them by solution or by ''stoping," and then sohdifies in great 

 masses of irregular or undetermined forms, these masses are called 

 hatholiths. 



Volcanic cones are familiar structures built up about the vents of 

 active volcanoes, and will be discussed under vulcanism. 



STRUCTURAL FEATURES ARISING FROM DISTURBANCE. 

 Inclination and folding of strata. — The original attitude of beds, 



biddings. Jour, of Geol., Vol. VI, pp. 704-710. 



