176 T, G. BONNEY AND F. T. S. HOUGHTON ON MICA-TRAPS 
Microscopic. Much decomposed, but apparently a crystalline mix- 
ture of plagioclase felspar and biotite, with grains of iron peroxide 
(some at least being magnetite) and a few grains of altered augite. 
The felspar is greatly decomposed, full in parts of earthy secondary 
products and colourless belonites, probably also of secondary origin 
(some, however, may be apatite), with occasional clear spaces of an 
isotropic mineral. The mica is frequently in hexagonal plates, 
rather unusually dark brown in colour, with a good many ferruginous 
enclosures—in one or two cases parts of the crystals are rendered 
quite opaque by them. 
The rock accordingly appears to be a kersantite. It forms a 
narrow dyke in the Coniston Grits. 
(12) Highest Dyke, Helm Gill, near Dent. 
Characters.— Macroscopic. A compact dark-grey ground-mass of 
felspathic aspect, containing numerous scales of black mica, varying 
up to about 0-1 inch diameter. Here and there is a small pinkish 
erystal resembling felspar. 
Microscopic. The transparent base exhibits with crossed Nicols a 
rather obscure microcrystalline structure, the better-defined crystals 
being prismatic in form, with a tendency to afan-like grouping. In 
this are some grains of magnetite, a great number of crystals of bio- 
tite, as usual, and a number of somewhat irregular pale-green grains 
of rather variable structure. They are often rudely defined at the 
exterior by small mica-plates and ferruginous grains, containing also 
small grains of both and numerous minute belonites. The green 
mineral is feebly dichroic and doubly refracting. Probably they 
have been augite or hornblende. Minute specks of calcite or dolo- 
mite abound in them, and are largely disseminated in the slide. 
A partial analysis shows that this rock is decidedly different in 
chemical composition from the next; this, however, is to a consider- 
able extent due to the much smaller amount of CaCO,. As the pot- 
ash is considerably in excess of the soda, we may group it with the 
minette-felsites. 
(13) Lowest Dyke, Helm Gill, near Sedbergh. 
Characters.— Macroscopic. Generally similar to the last, but slightly 
less conspicuously micaceous. 
Microscopic. This presents some differences. The ground-mass is 
more crowded with microliths of various kinds, so that it has amore 
granular aspect; and the greenish grains in it are larger, paler, and 
rather more clearly defined. They are occupied by an aggregate of 
dolomite and almost colourless serpentinous mineral, The ground- 
mass also is full of minute bright specks (dolomite?); it was 
evidently never distinctly crystalline, and is now, so far as can 
be seen, microcrystalline, suggesting orthoclase rather than plagio- 
clase. 
The following is an analysis :— 
