247 
LAKE COUNTRY ROCKS: MICROSCOPICALLY 
DESCRIBED. 
THOMAS TATE, F.G.S., 
Leeds: Hon, Sec. to the Yorkshire Boulder Committee. 
THE identification of the crystalline constituents of Eruptive Rocks 
by their optical behaviour when in thin sections under the microscope, 
is extremely simple. Each mineral, in virtue of its structure and 
composition, possesses characteristic optical properties by which it 
may be recognised. 
By transmitted light they appear either colourless, coloured or 
opaque. The coloured minerals may next be examined with the 
polariser only, when some will pass from light to dark tints as the 
prism is rotated (pleochroic), while others will remain unaffected 
veneers If the analyser be now added, those minerals which 
depolarise will give more or less brilliant chromatic effects as the 
polariser is rotated (anisotropic), while others will show no colour 
changes, merely remaining dark between crossed Nicols (isotropic), 
The commonest colourless sections are those of quartz, fe spars, 
leucite, nepheline, enstatite, olivine, apatite; and these are all 
anisotropic save leucite, which is dark between crossed prisms, and 
apatite, which usually continues bright. Muscovite, biotite, horn- 
blende and ferruginous enstatite are dichroic and anisotropic, while 
augite and diallage are non-dichroic but anisotropic, and all are 
coloured by transmitted light. Magnetite and pyrites are both 
Opaque, but, viewed by reflected light, the former is of a leaden and 
the latter of a brassy hue. 
The most abundant alteration products are chlorites, serpentines, 
calcite and opaque iron ores. The two former are green, only the 
first is pleochroic ; calcite is colourless, traversed by fine cleavage 
lines intersecting at an acute angle, and 
The following descriptions, the result t of the examination of over 
one hundred and twenty specially prepared slides, were drawn up 
primarily for those members of the Yorkshire Boulder Committee who 
possessed slides of such Lake District Rocks as are most commonly 
_ dispersed in the Northern Drift. They have also, it is said, been of 
some service to the members of the writer's petrological class. In 
. response to the wishes of both they are now willingly placed, in a 
more accessible form, at the service den 
[40]* Biotite granite: Snap. Quarts in round or — : 
grains, idiomorphic towards the felspars. Plagtoclase and 
scours a sia a aaa 
‘Naturalist, Angust 1892. | 
1893. 
