616 



INTRUSION OF TRAP BETWEEN STRATA. 



[Ch. XXIX. 



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 60 to 80 feet in height, represented by a, fig. 

 685, is in part wedged in between the rocks of limestone, 6, and 

 shale, c, which have been separated from the great mass, of limestone 

 and shale, d, with which they were united. 



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 white granular marble without fossils. 



Masses of trap are not unfrequently met with intercalated between 

 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 advancing fluid, if no verti- 

 cal rents communicated with the surface, and a powerful hydrostatic 

 pressure were caused by gases propelling 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 easily sepa- 

 rable, but in other cases adhering firmly together. The columns vary 

 in the number of angles, from three to twelve ; but they have most 

 commonly from five to seven sides. They are often divided trans- 

 versely, at nearly equal distances, like the joints in a vertebral column, 

 as in the Giant's Causeway, in Ireland. They vary exceedingly in 

 respect to length and diameter. Dr. MacCulloch mentions some in 

 Skye which are about 400 feet long ; others, in Morven, not exceed- 

 ing an inch. In regard to diameter, those of Ailsa measure 9 feet, 

 and those of Morven an inch or less.f They are usually straight, but 

 sometimes curved ; and examples of both these occur in the island 

 of Stafta. In a horizontal bed or sheet of trap the columns are ver- 



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



f MacCul., Syst. of Geol., vol. ii. p. 137. 



