112 EEPORT— 187'2. 



part of the irou stated in tlie analysis below probably occurs, tliougb a portion is 

 distinctly oxidized. 



The rock is quamed as a building stone at Tardree Mountain, where it sometimes 

 assumes a columnar structure. A specimen from one of tlie quarries was subjected 

 to an elaborate analysis by Mi-. E. T. Ilardman, of the Geological Survey of Ireland, 

 who gives the following as the constituents* : — 



Arutlysis of Trachyte Porphyry — Tardree Quarry. 



per cent. 



Silica 70 9ijO 



Alumina S'lOl 



Peroxide of Iron 2'.3-44 



Lime 7064 



Magnesia 0295 



Potash 4-2G2 



Soda 1-818 



Loss by ignition 2-102 



Phosphoric Acid trace 



99043 

 Specific gravity 2-4.3.3 



Hehitiom of the Trachytic and BasaUic liocki. — During a recent visit the author 

 ascertained with the greatest certainty the relative position of the tracliytic to the 

 basaltic rocLs of the district. In the first place, there does not appear to bo any 

 passage uf tlie two classes of volcanic rock into each other ; and each having becm 

 erupted and spread out in sheets, exhibits a la*ninated or bedded structure, which 

 enables the observer to determine their relative positions witlioiit much ditiicnity. 

 Both at Carnearny and Tardree Hills tlie trachyte porphyry may be observed to dip 

 boneatli the basaltic rocks of tlie siuTounding country ; and the observations made 

 here and elsewhere tended to show that of the two kinds of rock the trachytes are 

 the oldei'. 



On the other hand, both at Carnearny Hill and Scolboa, the trachyte seems to 

 have been penetrated by "necks" of later date filled with basalt, through which 

 some portions of the overh-ing basaltic sheets may have been erupted. We are not, 

 however, as yet in a position to say whether or not the trachyte is the oldest and 

 lowest of all the tertiary volcanic rocks of coiuity Antrim, as its base is nowhere 

 exposedt. 



The events which have taken place in the volcanic history of this locality appear 

 to have been as follows : — 



/■ At some early stage of the Miocene period large masses of tracliytic rocks were 

 poured forth from one or more vents, doubtless accompanied by craters as in 

 Auvergue. After probably a long interval of repose, new eruptions of basalt and 

 doleiite took place through fissures and small A"olcaiiic vents breaking through the 

 trachyte. These later eruptions of basalt may have enveloped the whole of the 

 tracliytic masses, which liave been subsequently laid bare by denudation. The denu- 

 dation of this region has been very great during Postpliocene and later times ; and to 

 it is due the obliteration of the actual craters of eruption over the whole volcanic 

 region of Antrim. 



Trachyte Porphyry of co. Doicn. — This rock is very similar in appearance and 

 constitution to that of Antrim, consisting of a greyish felsitic base with crystals of 

 sanidine and blebs of quartz. It is only visible at JBallyknock, about four miles west 

 of Hillsborough, surrounded on all sides by Lower Silurian rocks, but not very far 

 distant from the margin of the basaltic plateau of Antrim. There can be little 



* Jom-n. Eoy. Geol. Soc. Ireland, vol. iii. part 1, p. 27 (new ser.). 



t Mr. Hardman considers that tlie amount of lime .shown by the analysis proves that 

 the trachyte lias undergone some amount of metamorphosis or alteration, and considers it 

 probable that it is consequently older than the basalt of Antrim, a view which subsequent 

 examination in the field has enabled rae to verify. Dr. Bryco thinks the traehyts occupies 

 the position of the great oclire-beds. 



