TRANSACTIONS OF THE SECTIONS. 85 



and with the trap, and blends, almost insensibly, sometimes into one, and sometimes 

 into the other rock. 



In the centre of the district, about Ballybrood, is a small hill of the lower coal-mea- 

 sure shales, resting upon the upper limestone on one side, and on trap on the other. 

 This trap attains a thickness of 800 or 1000 feet, and is chiefly contemporaneous bedded 

 trap, but has some intrusive parts, which cut like dykes into the coal-measures. It rests 

 on a bed of ash, and at its eastern end, at Nicker Hill, near Pallas, the most curious 

 and complicated intcrstratifications of limestone, ash, and trap may be distinctly ob- 

 served. 



Beneath this upper trap comes a regular band of upper limestone, 600 or 800 feet 

 thick, surrounding the trap and coal-measures on all sides, and forming an oval basin. 

 From underneath this another great belt of trap and ash crops out, forming a cor- 

 responding outer basin, the dimensions of which are about 12 miles from E. to W., and 

 6 miles from N. to S. 



The lower limestone rises from underneath this, and undulates for some miles over 

 the adjacent country. Towards the N.W. some of these undulations are sufficiently 

 great to bring the upper limestone in again, underneath the present surface of the 

 ground, and with that largo parts of the lower trap and ash. There are thus formed 

 three considerable detached outlying basins of trap and ash, one round Cahernarry 

 and Roxborough, another about the eastern side of the city of Limerick, and a third 

 round Carrigagunnil. There are also one or two small exhibitions of similar rocks 

 towards the north, apparently on a rather lower horizon. 



The above igneous rocks are all bedded and interstratified with the limestones, 

 except in a few places, where they seem rather to occur as small intrusive dykes, 

 cutting through the other traps as well as the aqueous rocks. 



In many places the bedded traps become quite vesicular and scoriaceous, the vesi- 

 cles being often filled with carbonate of lime and other minerals, and thus forming 

 an amygdaloid. 



In some places these vesicular parts occur as irregular bands intermediate between 

 bands of solid, compact, or even crystalline trap, precisely resembling the figures given 

 by Sir C. Lyell of the junctions of different flows of lava on Mount Etna. 



There are, however, six other detached masses of igneous rock, five on the south 

 and one on the north of the basin above spoken of, which are clearly intrusive masses 

 rising up through the limestone, and not now connected with any overlying contem- 

 poraneous sheets of trap. It is probable that these mark the sites of the volcanic foci 

 or funnels, through which some of the sheets of trap flowed to the then surface, such 

 sheets, with the upper limestone including them, having been long ago removed by 

 denudation. It is also probable that similar small detached foci or funnels lie still 

 concealed beneath the areas occupied by the contemporaneous traps and ashes. 



One of these detached masses, called Knock Dirk (not the hill which is called merely 

 Dirk), is a true syenite, having crystalline particles of quartz mingled with felspar 

 and hornblende. 



It is difficult to give any precise name to the rock comprising the other masses. 

 Some of the traps, both intrusive and contemporaneous, would be commonly called 

 felspar porphyry, others greenstone, and others basalt. When the felspar porphyry 

 loses its distinct crystals of fclpar, it might perhaps be called felstone. Felstone, 

 however, as understood by the author, means a rock composed of a trisilicated fel- 

 spar, mingled with an overplus of silica in a state of paste; and it seems difficult to 

 suppose that sihcated rocks proceeding in a molten condition through and over such a 

 basic substance as the carboniferous limestone, should still contain anv uncombined 

 silica, except in the heart of a large mass like Knock Dirk. It would se'em, therefore, 

 adrisable to apply some other name, such as aphanite, for instance, to the compact 

 felspathic rocks above spoken of. In the absence of precise chemical analysis, which 

 the author regretted that he had been unable hitherto to procure, it had seemed better 

 to speak of all the igneous rocks collectively under the vague but sufficiently intelligible 

 designation of trap. 



Mr. Jukes also stated, that he was much struck with the very great resemblance 

 between these trappean ashes and some of the traps, and those which he recollected 

 to have observed in the volcanic islands in Torres Straits, where small detached vol- 

 canoes have broken through the coral reefs, and formed rudely conical accumulations 

 of stratified ashes containing lumps of coral limestone together with flows of horn- 



