Dr. Mac Culloch on the Geology of Glen Tilt, 333 



Since this paper was prepared for the press, there has been, 

 published in the Transactions of the Royal Society of Edinburgh a 

 description of the same place, drawn up from the joint observa- 

 tions of Lord Webb Seymour and Professor Playfair. Among 

 other differences, such as may be expected to occur in the obser- 

 vations of different individuals, and which are not subjects for dis- 

 jQUSsion, there are some which call for remark, since they involve 

 questions of nomenclature, the discussion of which may be useful 

 in the present uncertain state of that important preliminary to geo- 

 logical observation. It is much to be desired that all observers 

 should agree in the denomination of those rocks respecting which 

 frequent discordances arise, since without such agreement there 

 can be no prospect of a definite application of terms. 



Gneiss is described in that paper as of frequent occurrence. 

 With a few trifling exceptions the most remarkable one of 

 which 1 have described, I have no where observed any real gneiss 

 among the schistose rocks which form the left lidge of the 

 -valley or the great mass of stratified matter. These schistose 

 rocks, as far as my observations go, are clay slate, mica slate, 

 hornblende slate, and quartz rock. Gneiss is a rare rock in Scot- 

 land, and is principally to be found in the Long island, in Tirey, 

 Coll, Rona, lona, and Isla ; as well as on the mainland, in Glen 

 £lg, and Morven. I do not consider the rock which is of such 

 frequent occurrence between Blair and Loch Spey, already des- 

 cribed in this paper, as gneiss, since although the form is laminar 

 the structure is not foliated : nor have I observed that the gneiss 

 -of .Scotland any where alternates with mica slate, although it does 



