491 AILSA. GEOLOGY. 



are neither so perfectly defined nor so readily separable 

 as those of Staffa ; rather resembling those, more common 

 throughout the Western islands, where the forms appear 

 adhering, and occasionally, blended together. They are 

 of a large size, the detached masses which are capable 

 of being measured, reaching to six feet and upwards 

 in diameter, their sides and angles being both variable 

 and irregular. Although not jointed, they break, in most 

 places, at right angles to their axes. 



Proceeding further towards the north end of Ailsa, 

 this high range of columns terminates in a jutting and 

 small promontory, behind which is a recess containing 

 a cave. This is about twelve feet in^ breadth and thirty 

 in height, the depth being about tifty ; and it terminates 

 on the right hand in a kind of irregular dome. Though 

 not remarkable for its size, the position and form render 

 it extremely picturesque ; various ranges of columns at 

 different altitudes, and separated by irregular masses of 

 rock, surrounding it and stretching away to the northward 

 until they finally disappear. This part indeed presents 

 the most picturesque subjects that Ailsa affords; the 

 whole of the columnar face being visible at once if a 

 proper point of distance be taken, and the simpler 

 features and superior grandeur of the high ranges, 

 forming a beautiful contrast with the more intricate 

 disposition and complicated variety of the smaller.* 



Ailsa is composed of a single rock, no difference 

 being perceived between the amorphous and the colum- 

 nar parts. The whole mass must be considered as one 

 of the numerous modifications of the syenite of the 

 trap family. It consists of an almost uniform basis 

 of greyish compact felspar, occasionally tinged with a 

 brownish or reddish hue, having small grains of quartz 

 interspersed throughout. Together with that, it contains 

 black spots, formed of very minute particles of horn- 



* See Plate. X. 



