110 LYELL'S ELEMENTS OF GEOLOGY. 



Forcibly intruded Trap. 



whereas in other cases the fissure may give passage to a current 

 of melted matter, which may ascend for days or months, feed- 

 ing streams which are overflowing the country above, or are 

 ejected in the shape of scoriae from some crater. If the walls 

 of a rent, moreover, are heated by hot vapour before the lava 

 rises, as we know may happen on the flanks of a volcano, the 

 additional caloric supplied by the dike and its gases will act 

 more powerfully. 



Intrusion of trap between strata. In proof of the mechanical 

 force which the fluid trap has sometimes exerted on the rocks 

 into which it has intruded itself, I may 'refer to the Whin-Sill, 

 where a mass of basalt, from sixty to eighty feet in height, re- 

 presented by a, Fig. 97., is in part wedged in between the rocks 

 of limestone, b, and shale, c, which have been separated from 

 the great mass of limestone and shale, d, with which they were 

 united. 



Fig. 97. 



Trap interposed between displaced beds of limestone and shale, at White, Force, High 

 Teesdale, Durham. (Sedgwick.)* 



The shale in this place is indurated ; and the limestone, which 

 at a distance from the trap is blue, and contains fossil corals, is 

 here converted into granular marble without fossils. 



Masses of trap are not unfrequently met with intercalated be- 

 tween strata, and maintaining their parallelism to the planes of 

 stratification throughout large areas. They must in some places 

 have forced their way laterally between the divisions of the strata, 

 a direction in which there would be the least resistance to an ad- 

 vancing fluid, if no vertical rents communicated with the surface, 

 and a powerful hydrostatic pressure was caused by gases pro- 

 pelling the lava upwards. 



Columnar and globular structure. One of the characteristic 

 forms of volcanic rocks, especially of basalt, is the columnar, 

 where large masses are divided into regular prisms, sometimes 

 separable, but in other cases adhering firmly together. The 

 columns vary in the number of angles, from three to twelve ; 



*Gamb. Trans., vol. ii. p. 180. 



