4 Professor Hull — On Irish Granites. 



account for the formation of tubes of such, length, straightness, and 

 comparative thinness; but on showing the drawing which I had 

 made of them to Prof O'Reilly, he at once pronounced them to be 

 " tracks of gas bubbles," passing in certain directions through the 

 silica when in a viscous state. I have little doubt that this is the 

 true explanation of their occurrence. 



Gas cavities (?). — A remarkable constellation of cells in silica, 

 devoid of fluid bubbles, and which were,, therefore, probably filled 

 originally with gas, is represented in Fig. 10. In outline tlaey are 

 irregular, and when seen under a high power the uneven form of 

 the walls of each cell gives rise to bright points, shading off 

 laterally. When the light from the reflecter is allowed to pass 

 through the lower prism, the sides are seen to shade off internally 

 in a series of faint but delicately defined concentric rings of prismatic 

 colours, which art would fail to represent. 



Orthoclase. — The orthoclase occurs generally in formless patches 

 or in sub-crystalline forms, displaying with polarized light the 

 "cross-banded" structure, very strongly pronounced (Fig. 12); the 

 bands of colour intersecting at angles approaching 90°. This struc- 

 ture I have shown in a former paper to be due to planes of cells, 

 corresponding with those of cleavage, intersecting each other at 

 certain angles.' 



Triclinic Felspar. — Judging from the analysis of Dr. Haughton, 

 and the actual determination of the presence of albite in the granite 

 of Dalkey by Dr. Westropp, which is part of the same mass, I have 

 little hesitation in identifying the triclinic felspar which occurs in 

 the Ballyknockan granite with albite. There are several small sub- 

 crystalline forms in the slices, exhibiting with polarized light the 

 fine parallel lines, and bands by which the anorthic group of felspars 

 may be distinguished ; one of them is represented in Fig. 14. 



Mica. — The silvery-grey mica occurs in the slice almost colourless 

 to pale brown ; the black mica as a deep bronze. The former, with 

 a moderate power, is seen to be traversed by fine parallel lines 

 indicating the planes of cleavage. The bronze mica with polarized 

 light changes to black when the prisms are crossed ; the colourless 

 mica shows a rather faint play of colours on rotating the prisms. 



In fine — the most noticeable features in the structure of this granite 

 are the occurrence of a triclinic felspar, and the remarkable varieties 

 of cellular structure in the silica. 



EXPLANATION OF PLATE I. 



Fig. 1. Cross-section of felspar crystal. Granite of Aillemore. 



„ 2. Portion of crystalline grain of orthoclase, showing " cross-banded" structure. 



Magnified 25 diameters. Granite of Aillemore. 

 „ 4. Portion of orthoclase crystal, showing wavy-banded structure. Magnified 



25 diameters. Granite of Aillemore. 

 „ 5. Imperfect crystal of triclinic felspar (oligoclase ?), showing fine parallel lines. 



Magnified 150 diameters. Granite of Aillemore. 

 „ 6. Cells in silica containing fluid bubbles. Magnified 400 diameters. Granite 



of Aillemore. 

 „ 7. a. b. c. d. Fluid cavities with bubbles in silica of granite. Ballyknockan. 



* Paper No. 1, Observations on the Microscopic Structure of Irish Granites. 

 Geol. Mag. Vol. X. p. 195, 



