XIV. Supplementary Observation ou Quartz Rock,
made in 1814.
By J. Mac Culloch, M D. F.L.S. President of the Geological Society, Chemist to the Ordnance, Lecturer on Chemistry at the Royal Military Academy, and Geologist to the Trigonometrical Survey.
Having had an opportunity during the present summer of
adding some further remarks to the observations on quartz rock,
I have here arranged them as a supplement to the former papers, for
the purpose of extending the history and connections of this important
member of the more ancient strata. As I had not the means
of examining a very wide extent of country, I have not been able
materially to enlarge its geographical boundaries, yet the Society will
see that it constitutes a very considerable part of the country I did
observe. It is found in most parts of the valley of the Tumel,
which extends from the head of Loch Rannoch to the junction of
the Tumel with the Garry. Throughout this whole extent it alternates
with schist, sometimes micaceous, now and then containing
felspar and thus resembling gneiss in composition, but oftener argillaceous,
and very hard, from the large proportion of quartz which
it contains. As we approach the lower end of this valley the quartz
rock diminishes, while the schist increases, until the former at
length disappears. Beds of limestone are found alternating with it
throughout this course: it also contains porphyry, of which numerous
masses are to be seen on the sides of Loch Rannoch. These
