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THE GEOLOGIST. 



The eastern and southern slopes of the hill are gentle, but its more abrupt 

 north-western face forms a bold feature in the landscape. 



The rocks of which the core of the hill and this steep slope are formed 

 belong to the Silurian series, and consist of grey and bluish hard grits and 

 sandstones, interstratified with coarse conglomerates and fine flaggy and slaty 



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The eastern and southern slopes of the hill are formed of the Old lied Sand- 

 stone, which is represented here by whitish conglomeratic beds, dipping at low 

 angles from the higher Silurian ground towards and underneath the overlying 

 limestones of the surrounding plain. One of the most interesting points m the 

 structure of the hill was stated to be the disappearance of the Old Red Sand- 

 stone along its W. or steep flank, in consequence of the occurrence of a fault 

 runnino- along the base of the high ground at that side, nearly parallel to the 

 direction of its crest. By this fault a displacement of the rocks was caused, 

 amounting to more than 800 feet. This fracture does not seem to have been 

 the cause of the occurrence of a hill here, for as the Old Red Sandstone shows 

 a tendency to curve round the N. end of the hill, and actually does so at the 

 S., it seems likely that the hill was originally formed by an anticlinal, the axis 

 of which was arched at this place, and afterwards very obliquely crossed by 

 the fracture, along which the beds to the W. received an opposite or downward 

 curvature. Along the W. side of this fracture the limestone is let down so as 

 to come into juxtaposition with the Silurian rocks, and here, as is frequently 



Diagrammatic Cross-section of the Hill of Knockshigowna. 

 A, Fault ; B, Silurian ; c, Old red sandstone ; D, Carboniferous limestone. 



the case along lines of fracture in limestone beds, the rock is converted in 

 places into a (yellowish) crystalline dolomite. In other places where the 

 sequence is undisturbed, the lowest beds of the limestone series immediately 

 overlying the Old Red Sandstone are found to consist of the usual dark 

 earthy limestones, and cleaved, olive, calcareous shales, both being highly 

 fossiliferous. 



A peculiar group of red calcareous beds occurs in the Silurian rocks, close to 

 the unconformable boundary of the Old Red, and they may be traced along 

 their strike passing gradually from the ordinary bluish grey into a deep red 

 colour just, before they disappear beneath the Old Red Sandstone ; a circum- 

 stance which is in other places very common along the unconformable boundary 

 between this Old Red Sandstone and the adjacent Silurian, particularly when 

 the latter is composed of shaly beds. 



Fossils. — These calcareous red rocks do not appear to contain fossils, but in 

 the vicinity of a remarkable band of conglomerates in the Silurian, near Fairy- 

 mount-gate-lodge, and also in the conglomerate itself, and in the neighbouring 

 shaly flags, fossils were stated to have been found by the author and W. H. 

 Baily, Esq., F.G.S., by the latter of whom the following list was prepared : — 



