﻿424 THE CABI50NIFEK0US EOCKS AT LOTJGHSHINNT. [Aug. I908. 



The Loughshinny Black Shales now follow above the 

 Posidonoiny a-Jjimestones. They consist of black shale with bands 

 of soft black mudstone and many seams of chert, usually thin but 

 occasionally as much as 2 feet thick. Plant-remains are not 

 uncommon in some of the layers, while the thick cherty beds are 

 locally rich in casts of brachiopods and lamellibranchs. Lamelli- 

 branchs (Posidonomya BecJieri, P. memhranacea, Posidoniella Icevis) 

 and cephalopods (Thrincoceras, GlypMoeeras spirale) are found in 

 some of the shales. (For faunal lists, see p. 446. The absence 

 of Pterinojpecten ■papyraceus should be noted, as this form occurs 

 abundantly with Posidonomya BecJieri in other parts of the Dublin 

 district.) 



The principal exposure of these shales is in the south-western 

 portion of Loughshinny Bay, where they occur as a much contorted 

 syncline between anticlines of the Posidonomy a-Limestones. If 

 these contorted beds are measured from their base on the south, the 

 highest exposed bed proves to be about 105 to 110 feet above that 

 base. They reappear again farther north in Loughshinny Bay, close 

 to the village. 



The Posidonomya-Limestones ^ that intervene between these two 

 exposures of shale rise up into a succession of folds that are 

 beautifully exhibited in the cliffs. At first somewhat symmetrically 

 arranged like those on the south side of the bay, they rapidly become 

 more intensely folded as they are followed northwards until they 

 finally are somewhat overfolded (fig. 10, p. 425). One of the 

 anticlines (fig. 9, p. 425) shows a normal fold in its lower beds, and 

 passes into an overfold in the beds exj)osed higher in the cliff. At 

 the point where the Black Shales reappear the dip is reversed ; but 

 the beds soon recover themselves, and have an angle of dip smaller 

 than the normal. 



Some further instances of decalcification may be observed on 

 the shores of Loughshinny Bay, both north and south of the 

 main synclinal mass of the Black Shales. On the southern shore 

 a recess in the cliff shows an interesting stage in the solution of 

 the Posidonomy a-liimestones, as the limestone-bands may there be 

 traced as layers of ochre lying among a mass of black shale and 

 bands of chert. Among the ochre may be found lumps of little- 

 altered limestone. Again, to the north of the main syncline of 

 the Black Shales similar phenomena are exhibited, conspicuous 

 limestone-beds of the Posido7iomy a-Limestone group frequently 

 terminating among the more argillaceous rocks with which they 

 are interbedded, at times disappearing almost completely, at other 

 times being more or less replaced by ochreous matter. 



The calcareous shales in these localities are also decalcified into 

 ordinary argillaceous shale, and they then closely resemble the beds 

 of the Black-Shale group. Indeed, there are indications that the 

 Black-Shale group is itself in somewhat a decalcified condition, and 

 that it once contained calcareous shale and cherty limestone. This 

 view is suggested («) by the fact that the fossils in the chert-bands 



^ Some of the strata here exhibit false bedding. 



