Ch. XXIX.] 



STRUCTURE OF VOLCANIC ROCKS. 



483 



The shale in this place is indurated ; and the hmestone, 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 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 advancdng fluid, if no vertical 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 separable, 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 transversely, at nearly equal dis- 

 tances, 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 exceediug an inch. In regard to diameter, those 

 of Ailsa measure 9 feet, and those of Morven an inch or less.^' They are 

 usually straight, but sometimes curved ; and examples of both these occur 

 in the island of Staffa. In a horizontal bed or sheet of trap the columns 

 are vertical ; in a vertical dike they are horizontal. Among other exam- 

 ples of the last-mentioned phenomenon is the mass of basalt, called the 

 Chimney, in St. Helena (see fig. 633), a pile of hexagonal prisms, 64 feet 



Fig. 633. 



Fig. 634 



Small portion of the dyke 

 in Fii?. ""~ 



"Volcanic dyke composed of hori- 

 zontal prisms. St. Helena, 



high, evidently the remainder of a narrow dike, the walls of rock which 

 the dike originally traversed having been removed down to the level of 



* MacCul. Sys. of Geol. vol. ii. p. 137. 



