1858.] HARKNESS JOINTINGS. 93 



rated. The other, which is in cherty limestone, has only imperfect 

 N. and S. joints, but has the stratification very apparent. 



At Glasheen this is still better shown. On the west face of the 

 quarry here the point of contact of the two forms of limestone, 

 differing in their hthological nature, is seen. On the northern por- 

 tion of this west face, the ordinary limestone occurs cut up by the 

 three courses of jointing; but, this face being at right angles to the 

 more prevailing north and south series, this course is not visible, 

 although well seen on the north face of the quarry. On the southern 

 portion of this west face, the cherty limestones are devoid of visible 

 jointing, but show bedding in considerable perfection. The forms 

 of structure here seen are exhibited on fig. 7. 



The effect produced by the difference in the mineral nature of the 

 limestones, is also well seen in the lower portion of these strata, as 

 they occur at Hawksglen, near Raffeen, about a mile and a half 

 W.S.W. from Monkstown. Here the limestone is of the cherty kind, 

 with almost vertical strata, having an east and west strike and sub- 

 ject to coarse cleavage. Jointing is very imperfectly represented, 

 only slight traces of the N. and S. series occur, and very widely 

 remote horizontal joints. To such an extent is this the case, that 

 the limestone is wrought along the planes of its coarse cleavage, 

 which strike east and west ; and large flaggy masses are obtained by 

 this mode of working, a circumstance which could not occur had the 

 limestone in this spot been much intersected by divisional planes in 

 the form of joints. 



That the lithological nature of the limestone has a great effect on 

 the character and extent of the joints, is made stiU more manifest 

 by the analyses which were made for me by my friend and colleague 

 Dr. Blyth, and of which the following are the results. A siliceous 

 limestone, taken from the cutting of the Cork and Bandon Railway, 

 near the Cork Station, a locaKty already alluded to, and where the 

 stratified limestones are well seen, jdelded the following analysis 

 (the specific gravity beiag 2-677) : — 



Carbonate of lime 78*6 



Silica 14-6 



Carbonate of magnesia 5*5 



Iron 1-3 



100-0 

 In this limestone the jointing was imperfect, only widely remote 

 north and south joints making their appearance. 



A specimen of jointed limestone from the Gasworks quarry, about 

 a hundred yards N. E. from the cutting where the siliceous limestone 

 was procured, furnished the following results (the specific gravity 

 being 2-7): — 



Carbonate of lime 98-50 



Carbonate of magnesia 0-25 



Silica, iron, and loss 1-25 



100-00 



