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EY E. B. TAWNEY, F.G.S. 



Read Nov eynler bth^ 1874. 



/ 1 EOLOGISTS have long been aware of a remarkable Dyke on 

 ^ the Observatory Hill, which stands out like a wall from the 

 mass of Mountain Limestone. It is of Kew Red Sandstone age, 

 being composed of hard conglomerate, the so called Magnesian 

 Conglomerate. Everyone who walks up the hill on the Suspension 

 Bridge side, or who goes over the bridge, cannot fail to notice it. 

 It is ordinarily supposed probably to be part and parcel of the 

 Limestone, but an observant eye is arrested at once by its peculiar 

 position. True, on a cursory inspection, it may seem to consist 

 of Carboniferoiis Limestone ; but looking closer we notice that the 

 blocks of Limestone are separate, though cemented together into 

 one rock-mass by the red or yellow magnesian paste, and cemented 

 so firmly that the rock is tougher than the original Limestone. 

 This is the reason of its standing out like a wall, — it has resisted 

 denuding agencies better than the Limestone. Of course the 



