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IV. On a hitherto unobserved Structure discovered in certain Trap Rocks in 

 the County of Galway. By Robert Mallet, Esq., M.R.I. A. 



Read 10th April, 1837. 



The town of Galway is built upon part of a vast mass of trap rock, lying in, and 

 forming the embouchure of Lough Corrib, and which, running in a direction 

 nearly N. N. E., is lost beneath the sea in Galway bay at one end, and towards 

 the other may be traced to a considerable distance along the western shores of the 

 lake. This immense deposit appears to be a trap-dyke of the largest class ; it 

 separates the mountain-limestone of Galway and the neighbouring counties, on 

 the east, from the sienite of Cunnemara on the west. 



The limestone, at its junction with the trap, when not covered and obscured 

 by the sea or alluvial matter, is tilted up ; the otherwise nearly level strata making 

 angles of about eighteen degrees with the horizon. At one of these places, 

 namely, in the demesne of Renville, near Oranmore, about four miles from 

 Galway, a vein of cubical galena has been discovered, which offers a favourable 

 prospect to the miner, much "gossan" forming the " crop" of the vein. The 

 direction of the vein is nearly perpendicular to that of the trap-dyke. 



The trap appears on the other side chiefly to overlie the sienite ; but In some 

 places it mingles with it, as though by fusion in almost insensible degrees. Both 

 on the limestone and sienite sides, masses of each of these rocks are found enve- 

 loped in the trap ; — the limestone being much altered in hai'dness and colour, 

 its specific gravity and size of crystalline grain increased, and the rock occa- 

 sionally converted into something allied to basanite or Lydian stone. 



The existence of these imbedded masses of each of the neighbouring rocks, 

 with their alteration of character, and the tilting up of the limestone strata, would 

 seem to confirm the opinion that this deposition is a true trap-dyke. The 



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