PART I.] 



CULTURAL 



103 



rocks of eruptive origin, such as porphyries, are moulded into 

 the shapes which they now exhibit in the same way as granite, 

 and such also is the case with those sedimentary rocks which 

 consist to a considerable extent of felspar, such as many of our 

 gritstones. In these, however, a great uniformity of weathering 

 is caused by the regular lines of bedding which take the place 

 of the horizontal joints of the former class of rocks. The 

 vertical joints are similar in both. In igneous rocks, such as 



Limestone. 



basalt and greenstone, the jointing and fissuring is often of such 

 a kind as to give rise to very striking effects, very various in 

 their appearance, though probably closely allied in their origin. 

 Thus, from the simple dark brown, or black, trap, without 

 apparent structure, forming shapeless masses of a rounded, 

 somewhat unpicturesque, outline, there is but one step to the 

 bold semi-columnar escarpments of trap, which are so con- 

 spicuous in Northumberland and in many parts of Scotland ; 

 from these to the wonderful assemblages of rigid geometrical 

 pillars of Staffa and the Giant's Causeway, with all their 

 suggestiveness to rock-builders, the transition is shorter still ; 

 whilst in many parts of the three countries, we have examples 

 of trap weathering into a mass of many-coated spheres of every 



