EGG. GEOLOGY. 521 



porphyritic; being each about twenty feet in breadth. 

 The pillars of the columnar beds are by no means distinct, 

 and, from the smallness of their size, can scarcely at 

 a little distance be perceived, although they still commu- 

 nicate their general effect to the whole. They rarely 

 attain the diameter of two feet, and more frequently do 

 not exceed one ; while, in the western and detached parts 

 of the ridge, they are found even of three or four inches 

 in diameter. They are occasionally regular, although 

 varying in the usual manner in the numbers of their 

 angles, but are never jointed. On the contrary, the 

 course of each column must be considered as short and 

 independent, and they generally therefore terminate at 

 both ends in a thinner and prolonged shape, as if they 

 had been packed together in a parallel although irregular 

 manner while in a soft state, and thus suffered to compress 

 each other. From this cause they are often very irre- 

 gular in form ; being sometimes doubled, or even occa- 

 sionally cemented in greater numbers into a common 

 mass, so that the columnar shape entirely disappears. 

 They are in no case as easily separated from each other 

 as basaltic columns are ; whence, along the southern decli- 

 vity, immense fragments of the rock are to be seen which 

 have fallen from the summit and rolled for a mile or more, 

 even to the sea shore, without losing their integrity. The 

 top of the rock being formed of the ends of these columns 

 resembles a very irregular pavement, and under the lower- 

 most bed are seen some excavations beneath the over- 

 hanging rock, formed by the wearing of the softer sub- 

 stances on which the whole rests. 



The rock thus described is a pitchstone porphyry, and 

 is a substance of great beauty, the base being intensely 

 black, and the crystals consisting of glassy felspar thinly 

 disseminated. It is less brittle than the generality of 

 pitchstones, acquiring a harsh brown surface on weather- 

 ing ; and might perhaps with more propriety be considered 

 as intermediate between pitchstone and basalt. It con- 



