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NOTES ON SOME IGNEOUS ROCKS IN DOWN 

 ANDl ANTRIM.* 



By Miss Mary K. Andrews. 



(Abstract.) 



The following brief notes refer to rocks exposed in the bed of 

 the Glen River at Newcastle, Co. Down, to certain dykes on 

 the Mourne Coast, north of Glasdrumman Port, to a few of the 

 rhyolites of Co. Antrim, and to one or two points of interest 

 connected with its Basaltic Plateau. 



Beginning with the granite of the Mourne Mountains, 

 attention may be drawn to its well known resemblance in 

 miarolitic structure and other characteristics to some of the 

 granite of Arran, and to the probability that both are of 

 Tertiary age. Direct evidence is still wanting, but one of the 

 many points that support this inference, is that in its intrusion 

 into the surrounding grits and shales, the Mourne granite has 

 cut off a number of basic dykes, possibly belonging to the 

 Tertiary " Lower Basalts," and is itself penetrated by a less 

 numerous later series, probably representative of the " Upper 

 Basalts." In the first lantern slide (reproduced in Plate I.), 

 one of these older dykes is seen cut off by the granite. This 

 very interesting section occurs at an approximate height of 550 

 feet above sea level, and about 300 yards from the second stone 

 bridge in Donard Lodge Park. The photograph shows the 

 junction of the granite and Silurian rock in the bed of the 

 Glen River. The head of the hammer is on the line of 



* The paper was mostly illustrated by lantern slides from the author's geological 

 photographs, and by microscopic sections of specimens she had collected. 



