174 T, G. BONNEY AND F, T. §. HOUGHTON ON MICA-TRAPS 
as numerous more or less regularly formed plates of olive-brown 
biotite, frequently containing minute acicular, granular, or platy 
enclosures, being sometimes ferrite, sometimes perhaps smaller scales 
of mica, arranged generally along the planes of principal cleavage, 
giving rich colours with crossed Nicols. Nearly colourless crystal- 
line grains, now composed of calcite or dolomite, with a little viridite 
and other secondary products, are common. The general appearance 
suggests that they are pseudomorphs after augite ; and in one or two 
the angles of transverse sections of the crystal correspond fairly 
well with those of that mineral. We also find crystalline grains of — 
a ferruginous mineral, probably hematite. 
The following is an analysis :— 
EWiatter (eve tert ten semen 301 
COE ee ee a ein as 2:01 
SHO aeaasai pontine crowterdats 47°88 
AO 2 arti eeyiet es 19°14 
ClO une 4:33 
1 10 RR i NO tela 1:67 
VETTE AC are a ee 0°35 
CAO ee eee 6:16 
Nia OM rari ere 6°36 
EO ee rg metetee ge 5:54 
INGION ee ee 2°45 
98-90 
The analysis seems to place this rock with the minette-felsites, but 
there is evidently a very considerable portion of plagioclase felspar 
present with the orthoclase. 
The dyke now is not clearly exposed at the place where it is 
mapped by the Survey, so these specimens were collected from 
boulders on that spot, some blackish, others brownish in colour. The 
difference in the analyses is remarkable, and shows that they can 
hardly be from the same rock, but that there must be more than one 
dyke close by. The adjoining sedimentary rock belongs to the Kirkby- 
Moor Flags. 
(9) Dyke in River Lune, S.W. of Sedbergh. 
Characters.—Macroscopic. A compact dark ground-mass, full of 
crystals of dark-brown mica, up to about % inch in diameter. 
Microscopic.—The ground-mass shows great decomposition. At 
present it may be described as a compound mass of earthy-looking 
dust, often gathered into clots, and occasionally associated into 
crystal-like forms, interwoven, as it were, with colourless needles 
(some probably apatite), grains of opacite, flakes of mica, and sur- 
rounded by a colourless base. It is, however, more than doubtful 
whether any true glass is present ; and careful examination makes it 
appear probable that the whole has once been crystallized, though 
