THE 



GEOLOGICAL MAGAZINE. 



No. CVII.— MAY, 1873. 



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I. — Obsekvations on the Microscopic Structure of Irish 

 Granites. (No. 1.) 



By Professor Edwakd Hull, M.A., F.E.S. 



(PLATE IX.) 



[Bead before the Eoyal Geological Society of Ireland, 9th April, 1873.] 



Grardte of FirbogJi. Galioay Bay. — I propose, as time and oppor- 

 tunity permit, to give the results of observations on the microscopic 

 structure of Irish Granites, commencing with one from Galway. 



This is a porphyritic granite from the great tract of granitic rocks 

 which stretches westward from the town of Galway itself. It con- 

 sists of a base of silica, dull waxy felspar, and dark-green mica in 

 nearly equal proportions, in which are embedded pink or flesh-coloured 

 crystals of orthoclase. These last are sometimes half an inch in 

 length, and though generally occurring as individuals, are often pre- 

 sent as twins. The granite is foliated, and probably of metamor- 

 phic origin. 



Silica. — Silica occurs without crystalline form enveloping all the 

 other minerals. It is structureless, but full of cells which are visible 

 with a high power. With polarized light, and on rotating the upper 

 prism, the silica presents the usual gorgeous play of colours ; being 

 broken up into distinct patches of irregular form,, eacli refracting 

 different prismatic colours. Some of the patches show 'round their 

 edges parallel wavy bands of prismatic colours, marking out the 

 individuality of the patches, and indicating the manner in which the 

 particles consolidated in independent masses of various sizes — some- 

 times exceedingly small. The cellular structure of a portion of the 

 silica is shown in Fig. 4. These cells are often so minute that three 

 successive series are brought into the field upon changing the focus 

 of the microscope by means of the mill-headed screw, with a mag- 

 nifying power of 350 diams. Along with the cells are numerous 

 long " belonites," or " trichites," sometimes perfectly straight, and 

 stretching in all directions through the mass of the silica. With the 

 1-inch object-glass these can be generally observed, but with the 

 ^-inch and the No. 2 eye-piece, magnifying 350 diams., they are 

 very well brought out, sometimes in extraordinary numbers. Even 

 with this power their apparent thickness is not so great as that of 

 the finest needle, with an apparent length from an inch downwards. 

 In one or two instances they appear to be barbed, but this may be 

 owing to the meeting of two trichites at a point : there are also 

 VOL. X. — NO. cvir. 13 



