158 Pro/ Hull — On the Structure of Trap-rocks. 



are remarkably well formed ; and on the whole this specimen is one 

 of the best examples of the type-structure of the melaphyres of the 

 district. It bears a close resemblance to the rock from the " Lion's 

 Haunch," Arthur's Seat, near Edinburgh, of which I have had an 

 opportimity of examining a section, kindl}'- lent to me by Mr. All- 

 port, F.G.S. With the quarter-inch object-glass, well-defined cells 

 were observed in the augite crystals. 



(7). Specimen from Carrigogunnel. — This specimen comes from an 

 outlying district, separated from the Trap-bands by several miles of 

 limestone. Its position is about 4^ miles W. of Limerick, and it is 

 supposed to be contemporaneous with the " Lower Trap-band." The 

 trap itself is interposed between beds of ash.^ The base, as seen 

 with a high power (quarter-inch), consists of a highly crystalline 

 triclinia felspar, containing ill-formed augite crystals, rounded or oval 

 greenish grains (probably pseudomorphs after olivine)^ long prisms 

 of tricilinic felspar (probably Labradorite), and groups and grains of 

 magnetite. The felspathic base is throughout highly crystalline, 

 and is seen (with the quarter-inch object glass) to contain numerous 

 cavities, sometimes elongated, at other times oval or round. 



Olivine (?). — An oval-shaped large grain, filled with a nearly colour- 

 less material, but having numerous star-like radiating centres and 

 curved lines in green, resembles one of the figures of olivine 

 structure given in Dr. Zirkel's work on the structure of basalt.^ 



Felspar crystals. — The long prisms of triclinic felspar are very 

 like those of the Antrim dolerites, which are known to be formed of 

 Labradorite felspar. 



(8). Specimen from KnocTcca. — Dark compact trap (outlying boss 

 supposed to belong to the Lower Trap-band). — Colourless base with 

 slender prisms of triclinic felspar ; large quantities of magnetite in 

 crystalline grains, a few crystals of augite, and numerous grains and 

 patches of a greenish mineral. The augite grains are exceedingly 

 minute, and scarce. 



Olivine {?) . — Some of the greenish grains when examined with the 

 quarter-inch object glass show a radiating fibrous structure somewhat 

 resembling that of one of the specimens of olivine figured by Dr. Zirkel ; 

 and on the whole, although the crystalline form of this mineral is 

 often not at all, or only obscurely developed, I have little doubt that 

 these grains are olivine, or its pseudomorphs. 



(9). Mottled dark purple am ygdaloidal trap, Herbertstown (top of 

 Lower Trap-band). — Base a colourless glass, sometimes stained 

 brown, containing small prisms of triclinic felspar, and containing 

 cavities sometimes filled with a green structureless mineral (chlorite?), 

 but generally with calcite. 



The slice is not sufficiently thin, and is so much stained by oxide 

 of iron, that other minerals are not discernible. With polarized light 

 the banded structure and opalescence of the calcite, which fills the 

 fissures and cavities, is remarkably well shown. 



^ See Explanation to Sheet 143, p. 25. 



2 Untersuchungen iiber die Mikroskopische Zusammensetzung der Basaltgesteine 

 (1870). 



