deposition. They are accordingly very abundant in the sedimentary 

 rocks. 



I. n. ill. iv. 



SiO, 4471 ... 44-64 ... 43-47 ... 46-23 



Al,0 3 81-13 ... 30-18 ... 31-42 ... 33-03 



Fe.,0 3 4-69 ... 6-35 ... 4-79 ... 3-48 



CaO 1-09 ... ... 1-38 



MgO 0-90 ... 0-72 ... 1-13 ... 2-10 



K.,0 9-91 ... 12-40 ... 10-71 ... 8-87 



Na,0 1-27 ... ... 1-44 ... 1-45 



Loss 6-22 5-32 5-43 4'12 



99-92 99-61 99-77 99-28 



I. White mica from granite of Glendalough Valley, Co. Wicklow (HAUGHTON), axial 

 angle 70 4'. 



II. White mica from granite of Mt. Leinster, Co. Carlow (HATJGHTON), axial angle 72 18'. 



III. White mica from Three Rock mountain, Co. Dublin (HAUGHTON), axial angle 5 8'. 

 These micas are termed margarodite by HAUGHTON ; muscovite by ROTH. 



IV. White mica. Glenmalure, Co. Wicklow (Sullivan). Quoted from Rammelsberg's Mineral- 

 chemie, p. 516. 



Pyroxene. Monoclinic pyroxene occurs in certain granites, in many 

 granophyres and in some liparites. The common variety in the granites is 

 nearly colourless in thin section. It occurs as irregular grains or as 

 crystals having more or less definite form. Twinning on the ordinary plan 

 is very common. In the granophyres the augite is usually without form. 

 It may be pale brown to colourless or green. The latter variety occurs in 

 the Mull granophyres. Augite-bearing liparites are not known in the 

 British Isles. Rhombic pyroxenes appear to be rare in the acid rocks. 

 They occur, however, in certain pitchstones. They are usually found in 

 association with monoclinic augite and the relations of the two minerals 

 are the same as in the andesites. 



Hornblende. The hornblende of the acid rocks is similar to that of 

 the intermediate rocks and requires no special description. Quartz-diorites 

 shade into hornblende-granites in the most imperceptible manner. 



Iron-ores. These occur under the same conditions as in the inter- 

 mediate rocks. They are, however, less abundant. 



Apatite and Zircon occur almost constantly in 'granites. The latter 

 mineral when present in the ferro-magnesian constituents is usually 

 surrounded by a pleochroic border. It occurs in minute well-formed 

 crystals or as grains. Glass inclusions containing two or more bubbles, 

 sometimes more or less distorted, have been observed by CHRUSTSCHOFF in 

 the granites of certain localities. As zircon probably belongs to the 

 earliest period of crystal-development these glass inclusions are interesting 

 as furnishing evidence that granite has resulted from the consolidation of a 



