Extract

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 masses are sometimes truly porphyritic, containing crystals of felspar, while at other times they consist of an uniform reddish compact felspar, the usual basis of those porphyries which are found associated with the primary rocks. There is much difficulty in