156 Frof. Hull — On the Structure of Trap-rocks. 



veniently named " melaphyres," and which are the representatives in 

 Mesozoic and Upper Palfeozoic times of the more recent basalts and 

 dolerites. In general structure these Limerick melaphyi'es consist of 

 a glassy colourless felspathic base, in which are imbedded crystalline 

 grains of augite, magnetite (or titano-ferrite) , pseudomorphs of 

 olivine occasionally, with a considerable infusion of chlorite and 

 calcite. Sometimes large crystals of banded felspar occni', as in the 

 specimen from Herbertstown, In all cases, however, the base is a 

 glass, and the augite only occurs in distinct grains, and is by no 

 means abundant at any time. The dark green colouring of the 

 rocks is due to the generally large number of magnetite grains and 

 the abundance of chlorite or epidote. I now proceed to give a brief 

 account of the individual sliced specimens which I have examined : — 



(1). Specimen from Herbertstoivn (Lower Trap-band). — Base a 

 glass with cavities and tubes, passing into crystalline granular tri- 

 clinic felspar, with light-brown crystalline grains of augite, large 

 crystals of banded felspar, magnetite grains, and chlorite filling in 

 cavities, and in distinct grains. 



Augite. — Some of the crystals are very well formed, and inclose 

 magnetite grains. They are of a pale brown colour, much fissured, 

 and are locally numerous. 



Felspar crystals. — The large crystals porphyritically imbedded 

 show, with polarized light, the banded structure and the fine lines 

 characteristic of one of the triclinic group. 



Chlorite. — The chlorite occurs as filling in the interstices in the 

 base, and between the crystals, but sometimes as distinct grains, 

 with the rounded or oval form resembling grains of olivine. It is 

 not improbable it has replaced this latter mineral. In some places 

 it assumes the vermiform structure which is not uncommon, and it 

 has every appearance of being a "secondary" mineral, infiltrated into 

 the rock after consolidation. 



(2). Specimen from Boxborough (Lower Trap-band). — This is an 

 exceedingly interesting specimen, containing well-formed crystals of 

 augite, olivine, triclinic felspar, and (which is rare) hornblende, the 

 latter large and well defined ; the whole imbedded in a base blackened 

 by magnetite. The substance of the crystals is generally entirely 

 changed, and replaced by a yellowish mineral (epidote or chlorite) 

 and calcite. The base seen with the quarter-inch power (200 dianjs.) 

 is a colourless or yellowish glass, full of magnetite crystals, and 

 having long slender prisms of triclinic felspar imbedded therein ; 

 also nests and pockets of a yellowish-green mineral with radiate- 

 fibrous structure, probably epidote. This mineral — sometimes with- 

 out structure — frequently replaces the substance of the augite, 

 olivine, and hornblende. 



The specimen is remarkable for the large well-formed crystals of 

 hornblende, porphyritically developed. Generally the walls of the 

 crystals are filled 'in with secondary minerals, such as epidote and 

 calcite ; but there is one crystal retaining the original brownish- 

 fissured substance of the hornblende itself. This crystal enabled me 

 to apply the test of dichroism, which Tschermak and Allport have 



