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III, — On the Microscojm: Structure of a Gra7iitoid Quartz- 

 ]^orphyrij from Galway. 



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



Plate XLVIII. (Part of). 



Granitoid Quartz-porphyr]/, Attitliomasreagli, Co. Galway. — This 

 is a finely-crystalline granular rock, of a light reddish colour, con- 

 sisting of reddish and yellowish felspar, sihca in distinct grains, 

 and green mica ; in which are imbedded well-formed crystals of 

 reddish felspar. The position of this rock is near Salt Hill, about 

 a mile south-west from Galway, and it occurs amongst the great 

 mass of granitic and felspathic rocks which stretches westward from 

 that town.* 



If we assume that in all true granites the base is silica, 

 enveloping the other constituents, the microscopic structure of this 

 specimen tends to show that this rock is not a true granite but 

 rather a quartz-porphyry, as the base is seen to be felspar, in which 

 are enclosed all the other minerals, including the silica itself. At 

 the same time, from its granular character and the presence of all 

 the constituents of granite, it may be considered as bordering on the 

 region of the granitic series. 



The Base. — In order clearly to observe the base, a rather high 

 power (about 100 diams.) is required. It is colourless, but that it 

 is not " a glass " is proved by its action on polarized light, for on 

 rotating the analyzer it exhibits a waved or twisted structure with 

 a varied play of prismatic colours, in all probability due to imperfect 

 crystallization. The base is therefore clearly felsitic, and as it 

 is intimately interwoven with all the other minerals, it forms a 

 considerable proportion of the entire mass. With a higher power 

 (400 diams.) the erystalhne structure is very apparent, and inter- 

 mixed with the felsite itself may be observed much free silica 

 containing numerous cells. 



Felspars. — Orthoclase in numerous well-formed crystals is the 

 most abundant constituent of this rock, but along with this there 



EXPLANATION OF PLATE XLVIII. (Part of). 



Fig. 1. — Twin crystals of orthoclase — the larger of which shows a faint internal 

 structure parallel to the sides of the prism. The smaller is clouded, 

 and exhibits no structure internally. Magnified 25 diams. 



„ 2. — Orthoclase crystal formed round amorphous felsitic material. Magnified 

 40 diams. 



„ 3. — A double crystal of triclinic felspar and ortlioclase. 



„ 4. — Portion of a grain of silica containing numerous cavities, some with fluid 

 bubbles, others with solid materials (stone-cavities), others empty. 

 Magnified 400 diams. 



* See Map of the Geological Survey, sheet 105, with " Explanation." 



