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XIX.— NOTE ON THE AMYGDALOID AL ■ LIMESTONE OF 

 DOWNHILL, CO. DERRY. By PROFESSOR J. P. 

 O'REILLY, M.R.I.A. 



[Read, May 21, 1883.] 



Some months ago I received from Professor Hull a set of speoi- 

 meDS forwarded to him by Mr, Egan, of the Greological Survey. 

 He described them as filling cavities in sheets of basalt at Down- 

 hill, county Deny, and added, " It seems to be the curious mineral 

 mentioned by Portlock (p.' 215 of his Eeport) as occurring in that 

 locality." Mr. Egan further mentions, that a similar mineral is 

 to be found at Ballymoney, Co. Antrim, associated with calcedony, 

 and near Limavady, Co. Deny, where, in common with a hard, 

 flinty breccia (such as occurs between the chalk and the basalt in 

 various places along the great basalt escarpment), it appears to 

 have its origin in masses of chalk and flint caught up in the 

 basalt. 



Portlock thus describes the mineral : — " A curious mineral 

 occurs in soft amygdaloid at Downhill. It is oolitic in structure, 

 consisting of spheroids cemented together by pure white carbonate 

 of lime or by green earth. In the latter case it has much the 

 appearance of pudding-stone. The spheroids are yellow, whitish, 

 or greenish, and appear to be a mixture of the hydrocarbonates of 

 lime and magnesia. The small cavities are lined with drusy 

 crystals of pure white carbonate of lime : where exposed, the 

 cement, yielding first, assumes a mammillated appearance."^ 



He further adds, that the "greenish aragonite of Ballintoy and 

 the brownish aragonite of Downhill both contain a sensible amount 

 of strontian. 



The samples forwarded by Mr. Egan show the more usual 

 forms of occurrence of the mineral and the rarer forms. There is 

 also one showing " obscurely the manner in which the mineral is 

 often associated with aragonite." 



^ Geological Eeporfon Londonderry, p. 215. 



SCIEN. PROC. K.D.S. — VOL. IV. PAET IV. 



