John S. Flett — Orthite in Scottish Rocks. 389 



least eight recognizable crystals of orthite, besides smaller pieces 

 about the identity of which there might be room for doubt, and some 

 of these are fortunately so cut as to render possible not only a certain 

 identification of the mineral, but also a fairly complete account of 

 its optical properties. 



The granite of Fell Hill, Creetown, Kirkcudbrightshire, is shown 

 on the 1 inch Ordnance Survey Map as occupying a small triangular 

 area about a mile south of the town of Creetown on the shore of 

 Wigtown Bay. In the hand-specimen it is a fine white granite, 

 rich in black scales of biotite, with muscovite in subordinate quantity. 

 The microscope shows it is a true granite- with both muscovite and 

 biotite. The principal felspar is orthoclase, which is sprinkled over 

 with scales of muscovite, but microcline is also abundant, and an 

 acid plagioclase is present in fair quantity. Micropegmatite is almost 

 absent, and the rock shows distinct traces of shearing. The accessory 

 minerals are apatite, zircon, iron ores, sphene, epidote, and orthite. 



The epidote is very abundant, scattered through the whole slide 

 in small grains which are often grouped in irregular aggregates, but 

 do not show any evident idiomorphism. At the same time it seems 

 in every way probable tliat the mineral is largely of primary origin, 

 and not a product of the decomposition of the felspathic and 

 ferro-magnesian constituents of the rock. These are too fresh to 

 have produced epidote in such quantity, the biotite in particular 

 being in good preservation and not greatly exceeding the epidote 

 in amount. In several cases epidote is found in rounded grains 

 enclosed in biotite ; the mica is perfectly fresh, and around the epidote 

 are broad pleochroic halos, such as are usually seen about embedded 

 zircons. In such a case the epidote can hardly be other than primary 

 in origin. For the most part the epidote is scattered over the 

 felspar in such a way as to suggest its derivation from it. The 

 usual cleavage is well seen in the larger epidote grains, and when 

 they are elongated they usually have a straight extinction and yield 

 the emergence of a bisectrix or of an axis. The twinning on the 

 orthopinakoid which is so common in epidote was not observed 

 in any section. It is almost colourless or very pale yellow in 

 ordinary light, and the pleochroism is so faint as to be hardly 

 noticeable, ranging from colourless to a pale greenish yellow, 

 (a and yS = colourless, 7 = pale yellow.) The refraction and double 

 refraction ai-e both high, and in polarized light the colours are 

 biilliant, except in those sections from the orthodiagonal zone which 

 are nearly perpendicular to an axis, and such sections are by no 

 means uncommon. 



The orthite in thin section is deep brown in colour, and rather 

 resembles hornblende and biotite when cut in certain directions, the 

 more so as the outlines are sometimes hexagonal and resemble 

 closely those of transverse sections of an amphibole prism. From 

 hornblende, however, it is distinguished by its lack of cleavage and 

 its faint dichroism, while, unlike basal sections of biotite (with which 

 it would otherwise correspond), it does not yield the central emergence 

 of a uniaxial cross in convergent polarized light. The mineral is 



