JProf. Hull — On the Structure of Trap-rocks. 159 



(10). Specimen from KnocTcdirTc (an outlying mass, considered to 

 be a " root " or " neck " of one of the old vents). — This rock, as Dr. 

 Apjohn has observed, is different from the other Trap-rocks of the 

 district. Ho says, " On this hill is a close-grained greenstone, which 

 graduates into a clinkstone possessing sometimes an olive green, 

 sometimes a red, but more usually a brownish tinge." ' The specimen 

 in my possession from which the slice was taken is reddish in 

 coloixr, and of a crystalline granular structure. 



It consists of a colourless, or slightly stained felspathic base, with 

 crystals of orthoclase, and grains of silica. There are also a few 

 crystalline grains of magnetite, and irregular patches of green 

 chlorite. 



Silica. — As in the case of granite, the silica forms a paste in which 

 the felspar crystals, as well as some of magnetite, are imbedded, and 

 it was clearly the last to consolidate. Small cavities and cells may 

 also be observed with the quarter-inch object glass. Some of these 

 are in the form of long tubes extending in various directions through 

 the mineral. 



Felspar Crystals. — These I assume to be orthoclase from their 

 frequent rectangular form, and the absence of bands or fine lines 

 characteristic of the triclinic felspars. It was also to be expected 

 that in a rock so highly silicated as this, orthoclase would be present. 

 I could observe no augite crystals. 



On the whole, it is clear from the composition and structure of 

 this rock that it is altogether of a different species (or indeed genus) 

 from the traps above described, and approaches more nearly to the 

 composition of a trachyte, or clinkstone, than to that of a basalt or 

 melaphyre. It is the only one in which silica appears as an essential 

 constituent and in large proportions. 



(11). Specimen from Cahermarry. — Grlassy cellular base, amygda- 

 loidal, tinged deep-green by chlorite (?), with some small prisms of 

 triclinic felspar. Many of the cells are filled either with a green 

 fibrous mineral (chlorite?) or calcite, or both. (Figs. 6, 7, 8.) The 

 dark base is due to abundance of magnetite and chlorite. 



The points specially noticeable in reference to the above speci- 

 mens, which may be regarded as fair representatives of the Limerick 

 Carboniferous melaphyres, are the following : — 



1st. The glassy felspathic base with cells and tubes. 



2nd. The small quantity of augite, this mineral only occurring 



in the form of scattered crystals or grains. 

 3rd. The abundant infusion of chlorite, or more rarely epidote, not 



only filling in cavities and interstices between the crj'^stals, 



but also replacing, in many cases, the original minerals 



themselves (augite, olivine, etc.). 

 4th. The abundance of calcite, also due to percolation, and of 



secondary formation. 

 Antrim Miocene Basalts. — Considering that the rocks I have just 



1 Supra cit. p. 32. 



