HEAT. 



261 



217. 



temperature 200° F., would have its upper surface elevated 10 feet ; and that 

 a portiou of the earth's crust 50 miles thick, raised 600° F. to 800° F., might 

 become elevated 1000 to 1500 feet. Cooling would tend to reverse the result. 



(a) Contraction from cooling in case of fusion generally produces fractures 

 at right angles to the cooling surfaces; and in this way, "basaltic" columns 

 have been produced. Besides such transverse fractures, there frequently 

 exist longitudinal fractures along the middle or sides of dikes due to trans- 

 verse contraction ; and transverse fractures of 

 columns are very common. 



One of the most noted localities of " basaltic 

 columns " is that of the Giant's Causeway on 

 the northern coast of Ireland. The columns 

 (Fig. 217) are divided transversely and have 

 usually the upper surface of each section 

 slightly concave. In the columnar structure 

 the form is often six-sided, but live to nine 

 sides are common, owing to irregularities of 

 texture and cooling. 



Fig. 218 represents a scene from the coast of Illawarra, in southeastern 

 Australia, in which there are columns of two outflows, the nearer less per- 



Giant's Causeway. 



218. 





Basaltic columns, at Kiaraa, on the coast of Illawarra, New South Wales. D., Note-Book, '39. 



feet in form resting on horizontal stratified rocks, the other a larger outflow 

 in regular vertical columns five to eight feet in diameter. 



