312 
TATE: YOllKSHIRE PETROLOGY. 
mixture of pink grains with brown metallic flakes, having a some- 
what uniform texture. Throughout this groun(]-mass larger flesh- 
coloured crystals, up to one-hfth of an inch across, are sparsely 
scattered^ and, more rarely, clusters of hexagonal mica-plates each 
fully a quarter of an inch in breadth. To this macroscopical des- 
cription we must add, that a polislied face when viewed by reflected 
light, reveals numerous pale-grey bodies, presejiting in some cases 
the outlines of hornblende sections ; and the mica-plates of the 
gi'ound-mass are seen to have a fairly uniform arrangement parallel 
with the face of the dyke, the rock slices often splitting along sucli 
mica-courses in a provoking manner. The pink matter found in the 
spaces between the mica-plates is all that remains of the original 
felspathic constituent of the rock (fig. 24). 
Microscopically, the most prominent of the tluee chief mineral 
components, and the only one presenting idiomorphic contours is the 
brown mica. In each dyke of the Helmsgill group we find micas 
representing two different periods of consolidation in the life-liistory 
of the rock. Here the older are larger, mostly hexagonal-plates of a 
deep-brown tint, much corroded along their edges, and penetrated by 
the ground-mass (figs. 25-26). Sections transverse to the cleavage 
are of a pale-honey tint, contrasting sharply with the (jlive-green of 
the Ingleton micas, afs well as in _the absence of the darker lines 
formed by the ferric oxides concentrated between the cleavage planes 
and the consequent bleaching of the mineral. Like the hexagonal 
plates of the same age, their boundaries are exceedingly ragged and 
labyrinthine (fig. 27) ; often these sections have suffered mechanical 
deformation, the broken and displaced fragments lying contiguously 
(figs. 28, 29, 30). The younger, fresher-looking micas occur in 
rectilinear plates of a pale-brown tint, with ligliter margins and 
frayed ends Cfigs. 31-32), but this clear periphery is characteristic of 
the older micas in this rock rather than of the younger, the aggregated 
iron forming a black rim to the plate (fig. 26). They are strongly 
dichroic, and polarize with vivid chromatic effects, in this respect 
differing from the magnesiau micas present in Phillips's dyke, the 
rock this most nearly resembles. The resemblance is rather in the 
felspathic constituent, for in both rocks we have two generations of 
