Dr. Mac Culloch 07i the Geology ofGIett Tilt. 263 



referred some discrepancies and errors in the measurements of the 

 remarkable rocks which are seen in its bed. This cause is the variable 

 state of the water, which at times exposing more or less of these 

 projections in its bottom, alters their apparent magnitude as well as 

 their number. I have been rendered sensible of these differenees 

 by examining its course at different periods, but since some con- 

 fusion would have resulted from any attempt to correct the first 

 examination which I made, I have preferred giving it as it was first 

 recorded, with the caution against implicit reliance in the mea- 

 surements which I have now laid down. It is only necessary 

 t-o say that the observations were made in a kind of medium height 

 of the water, and that as the geological consequences are in no 

 wise affected by this sort of inaccuracy, I held it unworthy of 

 correction. The statement of the circumstance was however 

 necessary, lest the discovery of inaccuracies of this nature might 

 lead to doubts with regard to more important observations. 

 The measurement given in yards is merely a rude computation 

 from pacing, but it is fully adequate to the purposes In view ; since 

 neither the facts nor reasonings can in any way be altered by a 

 greater or less nicety of admeasurement. 



Descending therefore the course of the river from Poll Tarff, (See 

 Map plate 13) the junction of that stream with the Tilt, a large body 

 ■t)f granhe will be seen crossing it above the Shepherd's hut, and a 

 small granite vein may also be observed above it at a little distance. 

 The bed of the river from Poll Tarff to this point, wherever it is visi- 

 ble consists of dark blue limestone alternating with quartz rock. 

 At the distance of four or five hundred yards from the burn of 

 Aldianachie, a large rock consisting of a complicated mixture of 

 granite and schist crosses the Tilt again. I shall not here dwell 

 upon the circumstances which attend this mixture, as better oppor- 

 tunities of describing them will occur hereafter. 



