256 
Mr. Weaver on the 
perfect and continuous, but of the superior, the flanking strata only- 
remain, which approach more and more toward the vertical po- 
sition in the exterior. This structure may tend to explain the 
occasional appearance of vertical strata in parts of the limestone 
field ; as for instance, near Montrath on the north-eastern side, 
and again at Caher and Ardfennan bridge in the line of the 
Suire. 
§ 156. The average depth of this limestone tract in the in- 
terior is probably between two and three hundred feet ; if we may 
ground an opinion upon the elevation of the line of its outcrops, 
the protrusion of older rocks, both on the borders and in the in- 
terior of the field, the levels of canals, and the different elevations 
ascertained by the engineers employed in surveying the bogs, or 
in laying out new lines of navigation throughout Ireland. But in 
some parts of its extent, the limestone probably acquires a thick- 
ness of 500, 600, 700, and even 800 feet. The summit level of 
the Royal Canal is 808 feet above high water mark in Dublin bay, 
and the hill of Knock Drin near Mullingar, in the demesne of Sir 
Richard Levinge, is supposed to be 300 feet above this level ; but 
the surface of the limestone tract ascends considerably to the north, 
toward Knock Ion, the Bin of Fowre, and Lough Shillin. The 
summit level of the Grand Canal is 264 feet above high water 
mark in Dublin bay, and the level between Monastereven and Athy 
is between eighty and ninety feet lower ; and the Dunamase range 
of limestone hills is probably in the highest part 300 feet above 
this level. 
§ 157. The predominant colour of the limestone throughout 
the field is bluish grey, of various degrees of intensity ; but it is 
often tinged with black, and passes into deep black, particularly in 
fhose parts where it is interstyatified with numerous beds of slate 
