82 GEOLOGY or TRTCIITxVOPOLYj &C, [ClIAP. IV. 1. 



(for a profusion of thorny shrubs rendered the hillock inaccessible when 

 visited in 1861) the illuminated face of the tor appeared to coincide with 

 the north-south system of joint planes. 



In the Mallayanur tor this was unequivocally the case; indeed, 

 the north and south system of joints is strongly developed in many parts 

 of the granitoid gneiss region. 



Of very common occm'rence in this series of rocks are little veins and 



strings, and sometimes incrustations of a bright 

 Epidote. ^ \ _ _ * 



green mineral^ apparently Pistacite, a variety of 

 Epidote. . The frequency of occurrence of this mineral appears to be in 

 proportion to the amount of alteration the rocks have been subjected 

 to, and the presence of it is one of the reasons for supposing them to be 

 such increasedly-altered metamorphic rocks, and for not setting them down 

 at once as of plutonic origin. The three or four localities in which it 

 occurred in unusual abundance are mentioned below : — 



(a. J — On the south side of the high road, about 3 miles east of 

 Tiagar. fd.J — In a ridge of rock crossing the bed of the Guddalum 

 river a mile south-west of Marumvari. It is very abundant there, form- 

 ing coatings on every plane of jointing, and in some parts seems to enter 

 into the constitution of the g-neiss itself, which thus becomes a very 

 handsome stone, owing to the fine contrast of the bright green pistacite 

 and red-felspar crystals, (c.) — At Jumbay, 2 miles west of Mannalur- 

 pett, pistacite is well seen covering surfaces of joints in the most westerly 

 mass of the granitoid gneiss. 



Among the other members of the metamorphic series the pistacite 

 is very rare eastward of the mountains ; westward of them, and more 

 especially in the highly altered talcose gneiss occurring so largely to the 

 north of the Shevaroys, it again becomes a rather common mineral_ibut it 

 no where occurs in any large masses. 



A note-worthy fact in connection with the granitoid gneiss rocks ia 



the extremely small number of Trap -dykes occur- 



Absence of Trap-dykes. . , i i i • i i ^ i i 



ring over the whole area occupied by those rocks, 



(304) 



