XIV. Supplementary Observations on 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. 



[Read 17th February, 1815.] 



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 im- 

 portant 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 alter- 

 nates with schist, sometimes micaceous, now and then containing 

 felspar and thus resembling gneiss in composition, but oftener argil- 

 laceous, 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 nume- 

 rous masses are to be seen on the sides of Loch Rannoch. These 



