168 T. G. BONNEY AND F. T. S. HOUGHTON ON MICA-TRAPS 
bergh, and are described nearly as they would be met with in 
travelling from west to east. 
(1) Dyke 2 mile from Windermere Station. 
Characters.— Macroscopic. A compact ground-mass of ash-grey 
colour, enclosing numerous crystals of a dark-brown mica, most of 
which have a silvery lustre, apparently produced by their adherent 
films—probably a decomposition product. The mica crystals are 
commonly more or less hexagonal in form, and about 0-1 inch 
diameter. 
Microscopic. The rock, at first sight, appears to have a clear pale- 
brown glassy base, containing numerous microliths, generally aci- 
cular. Closer inspection reveals in the apparent glass more or less 
faint indications of a crypto-crystalline structure. With crossed 
Nicols this is rendered more distinct, innumerable microliths making 
their appearance. These show but faint light, and much of the field 
remains dark; but on rotating the stage new groups of microliths 
appear in the dark parts, while those formerly observed disappear. 
It may therefore be doubted whether any true glass now remains ; 
but it is highly probable that formerly such existed, and that the 
rock has been devitrified. 
In the above we have calcite or dolomite abundantly present, both 
in the ground-mass in fine granules, and in separate crystalline grains 
up to about 0:01 inch in diameter; brown mica (biotite) abundant 
in crystals of various size; granules of some iron peroxide, rather 
decomposed, aot rare; dolomite, and a nearly colourless hydrous 
mica (?) ; numerous grains composed of dolomite and serpentinous 
viridite. Among these we find sections corresponding closely with 
the familiar form of augite when cut perpendicular to ¢. 
The following is an analy sis of the rock :— 
IWiolbere Gren ci ee ccnree 2-59 
CON sae sce a eee 6°39 
SiO ie oe aegis tias 44-44 
BOR enced aNersae 17°85 
Fes OR roses open caat 4°82 
HeO ev. pene: 3°62 
Mi O seers ce uneemanentsc: tr. 
CAO ates ok eRe eae Tos. 
MgO’ og Setereuan cay ee TDM 
IE Oy tlie vals mage itta geekou 4-78 
ENE ON Ce eiamea ve iat ein 0-99 
100°59 
The proportion of K to Na is in favour of naming the rock 
minette-felsite ; but the percentage of SiO, seems low, and that of 
Fe and Ca high, for a rock containing orthoclase. It occurs intrusive 
