John S. Fleft — Orlhite in Scottish Rods. 391 



nsnally ia sharply idiomorpbic crystals. The epidote is never 

 idioinorphic, but from tbe parallel position of its principal (basal) 

 cleavage vpith one of the faces of the orthite the association is 

 undoubtedly that of parallel growth, between two isomorphous 

 minerals/ I failed to find sections parallel to the orthodiagoual 

 in which both minerals extinguished in the same position. In most 

 cases the extinctions were distinctly and even widely different. 

 Some of the orthite crystals are apparently free from any epidote ; 

 in others there is a very narrow rim of a clear mineral which could 

 not be identified, but is in all probability epidote ; and around the 

 twinned orthite this narrow border was also twinned, the composition 

 plane being identical for both centre and margin. Finally, in the 

 centre of the orthite crystals was frequently to be observed a pale 

 rose-coloured mineral with slight dichroism and lower double 

 refraction than either epidote or orthite. The orthite gives about 

 the same polarization colours as the couimon green hornblende of 

 granites, but this substance gave only pale greys and whites. It 

 may be a product of the decomposition of orthite, than which it seems 

 to be considerably softer, as it is often ground out of the interior 

 of a crystal in preparing the section, while the orthite and epidote 

 remain intact. It seems to vary somewhat in character, as between 

 crossed nicols it never has uniform colours and extinction, and 

 while irregularly cracked it has no definite cleavage. In one crystal 

 it forms the whole centre, being -24 millimetre in breadth ; around 

 it lies an orthite zone, sharply idiomorpbic, and varying from '01 to '04: 

 millimetre in bi-eadth ; around this, again, a clear zone, probably 

 of epidote, '02 millimetre broad, with its faces parallel to those of 

 the orthite. All three minerals are twinned on the same plane, but 

 they differ considerably in the obliquity of their extinction. The 

 nature of this mineral I liave been unable to determine. It may 

 be a manganese epidote, which I think is hai'dly probable, or 

 a product of the decomposition of orthite, although it should be 

 noted that it is always found at the centre and not on the surface of 

 the crystals. 



A second instance of an orthite-bearing rock is to be found in 

 the island of Sanday, almost in the extreme north-east of the 

 Orkneys. Here there is a large erratic, which from its size has 

 always attracted the attention of geologists visiting the county. 

 It is known as the " Savil " boulder from the farm on which it lay ; 

 but the proprietor, Major Horwood, has recently had it removed 

 and placed on the lawn at the front of the mansion house at Scar. 

 In his " Geognosy of Orkney," part i,^ Professor Heddle describes it 

 as consisting of " white, finely striated oligoclase, the crystals of 

 which are penetrated by fine filaments of actinolite ; glassy quartz, 

 in much smaller amount ; dark-green, finely foliated, lustrous horn- 

 blende, in well-marked crystals; very little of a pale-green mica ; 

 a minute amount of crystals of a pale-brown mineral, Avhich ma}', 



1 Hobbs, " Paragenesis of Epidote and Allanite": Amer. Journ. Sci., xxxviii, 

 p. 223. 



2 Mineralogical Magazine, 1880, p. 127. 



