Prof, Hull — On the Structure of Trap-rocks. 157 



both proposed for the determination of hornblende as against augite. 

 The mineral is more strongly dichroio than any specimen of horn- 

 blende I have yet examined, while the forms of the crystals are 

 of themselves sufficiently distinct for identification. But for this 

 specimen, I would have doubted the occurrence of hornblende 

 amongst these rocks. 



(3). Specimen from Bathjordan (Upper Trap-band). — Base a 

 colourless glass, sometimes tinged green, with multitudes of belonites 

 crossing each other in all directions, and numerous elongated cavities. 

 This felspathic base is darkened by crystalline grains of magnetite, 

 and contains crystals of augite and long prisms .of felspar. The 

 crystals of augite and felspar are generally replaced by chlorite (?). 



Augite. — The crystalline grains of augite are all more or less 

 replaced by chlorite ; but the forms are decisive, and are made very 

 plain by polarized light. 



Chlorite (?). — Light greenish mineral, with fibrous structure, filling 

 cavities or interstices ; as well as replacing the material of the augite 

 and some of the felspar crystals. 



Calcite. — With polarized light, the peculiar smalt-blue and opal- 

 escence of calcite is apparent, filling fissures and interstices in the 

 felspathic base. 



Cavities. — With the quarter-inch power, magnifying 200 diams., 

 numerous cavities are observable all through the glass base. Some 

 of these appear to be tubes, sometimes truncated obliquely ; others 

 are polygonal, or rounded. These are mixed up with numerous 

 "belonites," very similar in appearance to those of pitchstone, and 

 about the same size as those in a specimen from Ascension Island 

 with which I compared them. 



(4). Specimen from Hothjordan (Upper Trap-band, columnar). — 

 This specimen much resembles the last. The glass base contains 

 belonites, and cavities ; and along with well-defined brownish crystals 

 of augite are numerous pseudomorphs after olivine, filled with the 

 chloritic mineral which is so abimdant in these rocks. 



(5). Specimen from Balhjhrood (Upper Trap-band). — This is similar 

 to Nos. 3 and 4. The glass base contains belonites and numerous 

 elongated cells. The form of the crystals of augite are remarkably 

 well defined, but the substance of the mineral itself is often replaced 

 by chlorite and calcite. The crystals of magnetite often present 

 well-defined forms, but I could observe none resembling those of 

 olivine. 



(6). Specimen from Ballytrasna — Porphyritic (Upper Trap-band). 

 — I have two slices of this. The base is a glass with cells and 

 belonites in places, much charged with magnetite grains and crystals. 

 In this are imbedded well-formed augite crystals of a light-brown 

 colour, with imbedded grains of magnetite. One of these (a frag- 

 ment) is a large hexagonal crystal filled with a colourless and 

 structureless mineral, probably calcite. There are numerous large 

 crystals and groups of banded felspar, with a very fine appearance 

 under polarized light ; the augite crystals — showing a fine play of 

 golden bronze, and roseate colours on rotating the upper prism — 



