V. — Additional Remarks oji Glen Tilt. 

 By JOHN MCCULLOCH, m.d. f.l.s. 



CHEMIST TO THE ORDNANCE; LECTURER ON CHEMISTRY AT THE ROYAL MILITARY 

 ACADEMY ; AND GEOLOGIST TO THE TRIGONOMETRICAL SURVEY. 



[Read June 16, 1820.] 



LHE observations which were made on the structure of this valley in 1814, 

 appeared in the third volume of the Transactions of the Geological Society. 

 Since that period, I have had (in 1817) an opportunity of traversing the same 

 ground, and in somew^hat greater detail. In so doing I have found no occasion 

 to change the opinions then formed respecting its structure. On the contrary, 

 a greater intimacy with its parts and with their disposition, and an increased 

 practice in observation, have tended to confirm the remarks originally made. 

 A slight correction only is required, resulting from a much wider investiga- 

 tion than the former ; it respects gneiss, and will be introduced in its proper 

 place. 



Several additional circumstances have however occurred, which are not only 

 interesting in themselves, but will probably be acceptable to the Society, as 

 tending to render the account of this spot more complete. I need make no 

 apology for the detached form in which they are recorded, as they do not 

 admit of any useful arrangement. 



Granite Veins. — Since these veins have here been traced to the great mass 

 of granite, it would be interesting to know how far they extend from it, as 

 there are so few opportunities of pursuing the courses of granite veins to a 

 central mass. Those formerly described are immediately lost in the hills that 

 form the south-eastern boundary of the valley ; although it is not possible to 

 discover whether they actually terminate near the junction, or are prolonged 

 far into the superincumbent strata. About a mile, however, from Blair, on the 

 northern bank of the Tilt, I have since observed one which is about three 

 miles distant from the nearest visible granite : it is but a few feet wide, and 

 presents no pecuharity of structure. It is evident, however, that its apparent 



