﻿426 



DR. C. A. MATLEY ON THE 



[Aug. 1908, 



are mostly represented by casts, and (b) by the difficulty in fixing 

 the true base of the Elack Shales, the apparent base in localities 

 quite close to each other appearing to vary in horizon. Thus, the 

 highest bed of the main syncline of the Black Shales is, as stated 

 above, about 105 or 110 feet above the calcareous beds to the 

 south ; whereas, if measured from the calcareous beds of the anti- 

 cline on the north, the thickness seems to be considerably less, 

 and the Posidonomya-Lim.estone beds of that anticline appear to 

 pass laterally into beds indistinguishable from the lower Black- 

 Shale beds. 



Another interesting feature of the Posiclononu/a-Limestones of 

 Loughshinny Bay is found in the phacoidal calcite-veins exposed 

 in some abundance on the bedding-planes of the folded limestones. 

 These phacoids, of all sizes up to some 3 inches in length, are 

 usually grouped side by side in linear series, in such a way that 

 the long axes of adjacent phacoids are subparallel and transverse 

 to the line joining their centres (see fig. 11). Such a linear series 



Pig. 11.^ — ^Augen'-liJce craeJcs JiUed ivitJi caleite, in the Posidonomya- 

 Limestones, Loughshinny Bay. 



is often continued into a crack in the rock usually filled with calcite, 

 on the edges of which are frequently exhibited a number of broken 

 phacoids. The ends of the phacoids often tail out into the sur- 

 rounding rock as delicate hair-like threads, which are very well 

 displayed owing to the contrast of colour between the black shaly 



