196 *S'. Allport — Tertiary and Palceozoic Trap-roclis. 



The forms, however, are not so satisfactory as in one or two other 

 instances in which 1 have observed grains of magnetite in the mica 

 of granite, as, for instance, in the granite of Killiney Hill. The mica 

 in the section seldom shows well-defined crystalline forms ; and along 

 its margin is entangled with the other minerals, often in so ill-defined 

 a manner as to suggest the idea of a mineral in process of formation. 

 May not this be an appearance due to the metamorphic origin of this 

 granite itself? I am still, however, in doubt, whether the rich 

 bronze-coloured mica is of a different species from the colourless, 

 but equally twisted and scarred variety, seen under the microscope. 



EXPLANATION OF PLATE IX. 



Fig. 1 a b. — Portions of large felspar crystal, showing cross-banded structure ■with 

 polarized light, mica-llakes in lower part of the field; magnified 25 

 diameters. 



Fig. 2.— Portion of No. 1 magnified 200 diameters, showing cellular structure in 

 both directions ; also numerous little cells in the spaces interyening be- 

 tween the lines of cells. 



Fig. 3. — Cavity in felspar, with a well-formed crystal of mica (?) and a mica- 

 flake, together with little transparent prisms and grains ; magnified 350 

 diameters. 



Fjg. 4.- — Group of fluid cavities, minute prismatic crystals, and trichites in silica; 

 magnified 200 diameters. 



Fig. 5 {a). — Fluid cavity in silica; magnified 350 diameters, 



{b). — The same enlarged ; the fluid bubble resting on one side of the cavity. 



Fig. 6. — A spherular gas-cell in silica ; magnified about 1000 diameters. 



Fig. 7. — Spherular cells in silica, from quartz-porphyry of Lesmahago, Scotland; 

 magnified 55 diameters. 



Fig. 8. — Part of a mica-crystal, replaced by silica, and containing a spherular gas- 

 cell ; Lesmahago ; magnified 55 diameters. 



Fig. 9. — Crystal of triclinic felspar, probably oligoclase ; magnified 25 diameters. 



The slices were prepared by Mr. J. B. Jordan, of the Mining Record Ofiice. 



II. — Tertiary and Paleozoic Trap-rocks. 



By S. Allport, F.G.S. 



MANY Geologists will hail with satisfaction the accession of 

 Prof. Hull to the ranks of the few who appreciate the 

 importance of a microscopic examination of rocks ; and the communi- 

 cation from him which appeared in the last Number of the Geological 

 Magazine is the more welcome, as it contains a promise of further 

 researches in the same direction. 



In a new line of inquiry, every addition to the number of investi- 

 gators may be expected to give rise to new views, and produce 

 differences of opinion among those engaged in it. Prof. Hull's paper 

 may be cited as a case in point, for it appears that his observations 

 have already induced him to incline towards an opinion, which my 

 own investigations lead me to reject as untenable. 



In the paper referred to, the Limerick traps are described as con- 

 sisting of augite, felspar, magnetite and olivine, imbedded in a 

 glassy base, the augite occurring in small quantities in the form of 

 scattered crystals and grains. These rocks are then compared with 

 the Tertiary traps of the Antrim area, which are stated to consist of 

 felspar, titano-ferrite, and olivine, imbedded in an augitic base. It is 

 then shown that " the relations of the minerals are different in each 



