172) OW. A. Richardson—A Basic Dyke at Charnwood. 
original mineral was pyroxene. The groundmass contains a second 
generation of felspars, rather more acid than the phenocrysts, but 
is otherwise largely cryptocrystalline. 
The Greenstone.— Under the microscope a specimen from the dyke 
centre is seen to be holocrystalline and of medium grain. It is 
completely altered. With the exception of quartz none of the 
original minerals remain. The rock now consists mainly of quartz, 
chlorite, kaolinized pseudomorphs after felspar, calcite, and iron 
oxide. The structure is best made out by reflected light, when 
felted patches of lath-shaped felspars are seen to be surrounded by 
dark chlorite, showing that the original structure was ophitic or 
sub-ophitic. 
The chlorite is in ragged crystals and forms a kind of groundmass 
to the slide. It shows strong pleochroism and numerous inclusions 
of magnetite, and has the velvet-brown ultra-interference tints of 
penninite. 
The felspar pseudomorphs are chiefly kaolin, sometimes with a 
grain or so of epidote here and there ; but calcite replacements are 
not uncommon. 
The iron-ores are all changed to hematite and sometimes to 
leucoxene. 
Quartz is the most neers esting mineral. There is a good deal more 
of it than the analyses would lead one to expect. Little triangular 
fragments, similar to those seen in the Whin Sill, are clearly inter- 
stitial. Others take on the shape of felspar laths, and show a central 
plane of division corresponding to a Carlsbad twin- plane. The two 
halves extinguish differently. These larger quartz crystals may 
include calcite and chlorite flakes. Much of this quartz is doubtless 
secondary. Strain shadows are shown, but not so marked as in the 
quartz of the porphyroid. 
In one slide only there occur some irregularly oval rings of 
magnetite, enclosing carbonates very suggestive of pseudomorphs 
after olivine. The occurrence of these pseudomorphs is another 
reason for considering that the quantity of original quartz was 
probably small. 
It would thus seem certain that the rock forming the centre of 
the dyke was originally an ophitic dolerite, and possibly a quartz- 
dolerite. 
The marginal phase is very fine-grained. Many of the specimens 
obtainable are badly weathered and reveal little under the micro- 
scope. The dyke has again been quarried more recently and better 
marginal material has been made available. A slice cut from one 
of these specimens contains minute felspar laths, the larger of which 
are simply twinned. They appear to be oligoclase. The groundmass 
has a general greenish tinge and is cryptocrystalline. It is quite 
indeterminate, and was doubtless originally glassy. 
The Contact—Slides cut at the junction show that the contact 
is marked by a film of iron oxide, aa, Fig. 1 
