OF CERTAIN STRUCTURES FOUND IN MANY ERUPTIVE ROCKS. 213 



basalt, probably because this rock has been superf used, and is therefore 

 sometimes called basaltic structure. Sheets and- dikes of this rock are 

 often found composed wholly of regular prismatic jointed columns, 



New South Wales (Dana). 



closely fitting together, varying in size from a few inches to a foot 

 or more, and in length from several feet to fifty or one hundred feet. 

 When these columns have been well exposed on cliffs by the action of 

 waves, or on river-banks by the erosive action of currents, or even by 

 atmospheric disintegration, they produce a very striking scenic effect 

 (Figs. 187, 188). In Europe the Giant's Causeway, on the coast of 

 Ireland, and Fingal's Cave, in the island of StafTa on the west coast of 



Fig. 188.— Basaltic Columns on Sedimentary Rock, Lake Snperior (after Owen). 



Scotland, are conspicuous examples. In the United States we have ex- 

 amples in Mount Holyoke, on the Connecticut River ; in the Palisades 

 of the Hudson River ; in the traps on the shores of Lake Superior ; 

 and especially in splendid cliffs of the Columbia and Deschutes Rivers, 

 in Oregon. 



Direction of the Columns. — The direction of the columns is usually 

 at right angles to the cooling surface. In horizontal sheets, therefore, 

 the columns are vertical, but in dikes they are horizontal (Fig. 189). 

 A dike left standing above the general surface of country sometimes 



