104 Proceedings of the Royal Irish Acadern?/. 



ortlioclase) + ripidolite]. They are light greenish or ptu-plish to bright 

 green in colour ; tough but weather more or less freely ; generally have 

 a scaly or mealy aspect ; some, however, are compact {dAabau-aplmnyte). 

 They break with an uneven to a subhackly fractui'e, and fuse more or 

 less readily before the blowpipe. Some diabase is magnetic, due to the 

 rock containing pyn'hotite as an essential {^magnetic or pyrrliotitie 

 dialase) ; a common variety is highly micaceous yrnicaceous-dialase), while 

 others are both micaceous and pyiitic, the latter often weathering into 

 a mica-feiTuginous sand which may be magnetic. 



Llandovery JJliinstone. — Bedded doloiytes in the TooiTQakeady con- 

 glomerates. 



Post-siJurian Wlumtone. — Some of these rocks apparently are very 

 similar to the diabase already described, while in others lu-alite replaces 

 the diallage (uralitic-diahase). Some of the uralitic-diabase seems in 

 places to gi'aduate into a rock apparently a eiu-yte or basic felstone. 



Carboniferous ('•') Wliinstone. — The age of these rocks has not been 

 positively proved, *•' neveilheless, it is evident that they are newer than 

 the rock just described, as in all places where rocks of both gxoups are 

 associated together the diabase is displaced and cut by these whinstones ; 

 moreover, they seem to be newer than all the felstones. They are 

 doJerytes, probably melapliyres [pyroxene + felspar (not orthoclase) with 

 or without some amphibole]. These doleiytes are from granular to 

 compact, have a glisteiing resinous lustre, are dark-coloui-ed (blackish, 

 blackish-brown, and dark olive), and tough. The compact, homo- 

 geneous varieties 'yineJaphyre-aflianyte') have a conchoidal fi-acture, 

 while the granular rocks break fi-om uneven to subhackly. Some 

 of the melaphyres are porphyritic, while others are more or less 

 micaceous {rnicaceous-meJaphyre). In a few were obseiwed siliceous 

 blebs (opal ?) ; all fuse readily before the blowpipe. T^'hen in mass the 

 outside shell of these dolorytes has a peculiar tuffoid aspect and con- 

 tains few or many zeolites. f All the melaphyi'es, but especially the 

 aphanytes, usually weather fi'eely, the latter neaiiy always being in 

 naiTow dykei?. The fi'ee weatheiing seems in part due to the structiu'e 

 of the rocks, many being very jointy, causing them to break up into small 

 angular fi'agments. This reticulated structui-e appears to be caused by 

 a i-ude columnar structure perpendicular' to, and a platy stmctui'e 

 rudely parallel to the walls of the dykes. In some of these rocks there 

 is a spheroidal stinicture combined with the platy (fig. B, PL 9), while 

 in others the fi'agTaents into which the rock breaks, ai'e found to con- 

 sist of consecutive layers. In one locality, part of a dyke was found to 

 be amygdaloidal ; this stiiictur'e occui'red along a joint in the dyke (see 



* My colleague, E. G. Symes, F. E. G. S. I., kc, «tc., has proved that -whin- 

 stones in the neighhom-hood of Castlebar, Co. Mayo, exactly similar to those now 

 being described, are of carboniferoiis age. 



t These tiifPoid poitions may be due to the rock mass being protmded into 

 water ; consequently, the outer part would be liable to be pulverised and disinte- 

 grated (see page 113). 



