— 
W. A. Richardson—A Basic Dyke at Charnwood. 171 
Nor do they show very close agreement with respect to those con- 
stituents with any individual dacite, the analysis of which is 
available. The rock would seem to be a rather basic member of the 
group. There is, however, a tonalite from Lake Avio, Tyrol,! whose 
analysis compares much more closely with that of the porphyroid. 
Lhe Dyke. — The dyke shows a chilled marginal phase and 
a crystalline centre. I give in Table II analyses from the best 
obtainable specimens of each phase. It will be seen that they 
represent, as Professor Bonney suggested, a dolerite. The analysis 
from the centre compares with the quartz dolerite of the Whin 
Sill, quoted in column III. 
TABLE No. If. 
I II Il 
SIO sunur 62°21 50°43 alls22 
PANS Ost Mul Mi 16:09 14:44 14:06 
INS AO) 0 38 13°00 12°80 4°32 
Fe Olea! n.d. n.d. 8:73 
MgO aie 4-15 4°35 4°42 
CaO Ee 5:44 11:12 8°33 
I OMia cs 3:09 104 (2°55 
VECO Reena n 0-40 0-45 
IMINO) 6) 3556 n.d. n.d. — 
AMOS ise n.d. n.d. 2°42 
Ignition, etc. 5:00 5:00 2°37 
Motel. 99°38 99°63 99-67 
Sp. Gr. 2°77 2°81 
I. Greenstone Dyke, Peldar Tor Quarry, Whitwick. Marginal Phase. 
TI. us mY AS mi Centre. 
Ift. Quartz Dolerite, Whin Sill, Cauldron Snout. (J.J. H. Teall.) 
It will be noticed that the marginal phase is rather more acid and 
of lower specific gravity than the centre of the intrusion. This 
may possibly be due to a slight amount of marginal assimilation, for 
microscopic slices at the contacts show portions of the porphyroid 
partially absorbed. The specimen analysed was obtained quite close 
to one of the contacts. 
Il. PETROGRAPHY. 
The Porphyroid.—tittle need be said about this rock, since it has 
been minutely described by Professor Bonney.2 The phenocrysts 
consist of corroded and fractured quartz, plagioclase, and aggregates 
of fine epidote grains, forming pseudomorphs after a ferromagnesian 
mineral. One of these preserves an almost perfect octagonal cross- 
section with prism faces nearly at right angles, indicating that the 
1 Tddings, Igneous Rocks, vol. ii, New York, 1913, p. 142, No. 2. 
* Bonney & Hill, Quart. Journ. Geol. Soe., vol. xxxvi, p. 342, 1880. 
