520 EGG. GEOLOGY. 



larity, but at the eastern extremity, where the sides are 

 most parallel and the regularity greatest, it does not seem 

 to exceed 100 yards; towards the western extremity it 

 is considerably less. The height, as measured by the 

 plumb line, was 470 feet ; but the obliquity of the base 

 renders it impossible to give a just conception of the 

 nature of that measurement, without the assistance of 

 a diagram showing the mode adopted.* 



Nearly the whole of this rock is formed of columnar 

 beds, which vary considerably in different parts, both 

 in their size and disposition : they are most regular toward 

 the eastern end. Here the columnar forms are either 

 vertical, or at least placed in an upright direction, with 

 the exception of one oblique bed visible at the very 

 extremity. In proceeding to * the west, both the arrange- 

 ment of the beds and the directions of the columns vary ; 

 the latter being in many places inclined, horizontal, or 

 disposed in curvilinear directions ; while, in some parts, 

 even the columnar form disappears. At the eastern 

 extremity five distinct beds are to be traced. The three 

 lowest of these are columnar and formed of the porphyritic 

 rock about to be described ; the fourth is of common 

 trap, and the fifth and uppermost resembles the lowest. 

 Toward the western end however there are three trap 

 beds interposed among the others, the point at which 

 the change takes place not being easily determined. They 

 are readily distinguished by their non-columnar aspect, 

 as well as by their tendency to decomposition ; in conse- 

 quence of which they form deep furrows in the face of the 

 rock. They are of a grey colour, with a texture inter- 

 mediate between basalt and greenstone, and occasionally 



* The diagram represents the end view, and the point assumed for 

 measurement is that where the absolute perpendicular altitude is greatest. 

 It will readily be perceived at the same time that the real altitude is 

 more considerable, since it lies between the two horizontal lines AC. 

 Adding for the space A B 120 feet, the total height is 470 feet, that of 

 th portion actually plumbed B C being 350. See Plate XXIII. fig. 6. 



