154 -P'^'o/". Hull — On the Structure of Trap-rocks. 



the results of their labours to the world. As representing the Geolo- 

 gical Survey in this country, I may mention that I have been fully 

 alive to the great benefits to be derived from the use of the 

 microscope, and nearly one hundred specimens of sliced rocks have 

 already received a certain amount of scrutiny, and the results have 

 been noted. These results vsrill be from time to time inserted in the 

 Explanatory Memoirs which are published along with the Geological 

 Maps; and the Memoir issued last year (1872) on the Geology of 

 portions of Counties Down and Armagh, near Banbridge,^ will be 

 found to contain the first of these published series of observations 

 on the microscopic structure of some of the rocks of the district. 



Amongst the rocks which I have had opportunities of microsco- 

 pically examining, the basalts and dolerites of the North of Ireland 

 and the older melaphyres of County Limerick have received some 

 attention ; and I purpose in this paper to institute a comparison 

 between these two classes of rocks, which may be considered as the 

 representative augitic lavas of the Miocene and Carboniferous periods 

 in Ireland. 



Authors of Papers on the Limerich Trap-rocks. — The geological 

 structure of the Limerick Carboniferous basin has been described by 

 several writers. Of these may be specially mentioned Mr. Weaver,^ 

 Dr. Apjohn,^ Mr. Ainsworth,* Sir E. Griffith,* and Messrs. Jukes, 

 Kinahan, and Wynne." From the description, often exceedingly 

 full, which these authors have furnished, it is now understood that the 

 trap-rocks of the Limerick Carboniferous basin are themselves of Car- 

 boniferous age. As Mr. Kinahan observes, these traps and associated 

 ashes form two great deposits, occupying well-defined geological 

 horizons ; the lower coming in a little above the base of the Lower 

 Limestone, immediately over the cherty beds that are taken as the 

 division between the Lower &nd Upper Limestones ; and the upper, a 

 little below the basal shales of the Coal-measures.' There seem to 

 have been two principal out-pourings of lava from sub-marine or 

 subaerial vents, accompanied by eruptions of ashes and agglomerates, 

 the materials having been strewed over the adjacent seas ; and Mr. 

 Jukes suggests that some of the detached bosses of trap are to be 

 regarded with great probability as the volcanic foci or "necks" 

 of these old Carboniferous craters.^ 



Names and Mineral Characters of the Rocks. — Though these trap- 

 rocks present considerable variations in appearance and texture, they 

 are more or less basic (with one exception), and probably augitic. 

 The terms they have received, however, from various authors are 



1 Expl. Memoir to accompany Sheet 48 of the Maps of the Geol. Survey. 



2 " On the Geological Eelations of the East Coast of Ireland." Trans. Geol. Soc, 

 Lond., vol. v., by Thomas Weaver, M.R.I. A. 



3 «' Observations on the Trap District in the Co. Limerick." Journ. Geol. Soc. 

 Dub., vol. i. p. 24 (1832). 



* A Further Account of the Limerick Trap-rocks, by William Ainsworth, (1833) 

 ■with Map. 



5 Geological Map of Ireland, vrith reference (1839). 



« Explanations of the Geological Survey Maps, Sheets 143, 144 (1860). 



"> Ibid. Sheet 44, p. 24. ^ Ibid. Sheet l43, note p. 10. 



