626 



GEOLOGY. 



globular, pointed, or indeterminate, as in figs. 6, 7, 8. The appearance of stratification is 

 sometimes presented, but the deception is readily detected, not only by the nature of the 

 rock, but by the lines which seem to indicate it, running into each other, and extending 



only short distances, as i 



P<g- 9- 



9. But the phrase, indeterminate structure, cannot in the 

 strictest sense be applied to several igneous rocks, which, 

 without stratification, assume beautiful and distinct forms, 

 caused by the particular circumstances under which they 

 were produced. The most important of these forms is the 

 columnar, a structure which particularly characterises basalt, 

 and is occasionally exhibited by porphyry and greenstone. 

 Masses of basalt, divided into columns or prisms, occur at 

 Staffa, at the Giants' Causeway in Ireland, and in various 

 volcanic districts, which have more the appearance of works 

 of art than of nature, and constitute some of the most striking scenery of the globe. The 

 columns have three, four, five, 'six, and eight sides, but are commonly pentagonal, and 

 are so closely compacted together, that though perfectly separable, there is no perceptible 

 space between them. Figs. 10. and 11. represent these ranges, vertical and inclined. A 



remarkably fine example of 

 columnar basalt occurs upon 

 the banks of the little river 

 Volant in France, a view of 

 which is given. Sometimes 

 the basaltic columns are not 

 continuous, but consist of a 

 number of short pieces placed 

 upon one another. In North 

 America, the columnar ar- 

 rangement is very commonly 

 assumed by greenstone, the 

 rock which forms the Salis- 

 bury Crags, near Edinburgh. 



A spot on Mount Holyoke in Massachusetts, which has been called Titan's Piazza, 

 exhibits a group of greenstone columns, which hang over the observer's head, the pro- 



