TIREY. GEOLOGY. 49 



trap, and porphyry, only, are unstratified ; while the others 

 possess characters of stratification almost always very un- 

 equivocal, although in many cases attended with marks of 

 posterior derangement. It is possible that the masses of 

 limestone thus found in gneiss have once been stratified, 

 and that they have suffered some posterior changes by 

 which the appearances of this disposition have been obli- 

 terated. In illustration of this opinion I may point out 

 the state of the white marble of Sky, hereafter described ; 

 which, though at present as shapeless as the limestones in 

 question, has been once undoubtedly stratified, since it 

 forms portions of a series of parallel strata containing 

 organic remains. Similar illustrations may be drawn 

 from the nature of the limestone in the Isle of Man, which 

 I have described elsewhere ; as well as from many well 

 known rocks of this family, which, although now without 

 the slightest mark of stratification, give indications of their 

 having been originally thus formed, from their containing, 

 like those of Sky, org r anic fossils. 



The nodule of limestone which constitutes the marble 

 of Tirey under review, appears to be an irregular 

 mass of about 100 feet in diameter, and is surrounded 

 on all sides by gneiss. The contact is always very defi- 

 nite, yet the quartz seems to be the substance most gene- 

 rally in union with the limestone. Notwithstanding this 

 defined contact, it occasionally contains imbedded lumps 

 of granite or gneiss similar to those which occur in the 

 limestone of Glen Tilt. These are always visible at the 

 surface, from their superior power of resisting the action 

 of the atmosphere. This limestone is of a reddish hue, 

 varying from a high flesh-colour through pink to nearly 

 white, and from a muddy crimson to a dull purple ; often 

 also with a greyish aspect bordering on blue. It is of a 

 very fine splintery fracture and smooth grain, precisely 

 like the marble of lona. Though no where bedded, 

 nor capable of being raised in parallel-sided masses, yet 

 after exposure to weather it not unfrequently splits into 



VOL. i. E 



