844 THE OCCURRENCE OF StLTTRIAN [?] ROCKS [Aug. I9OI, 



Scottish Highlands, and with reference to theoretical questions in 

 tectonic geology and metamorphism. If anywhere the Arenig 

 Group really shades off without a break into the Highland Schists, 

 we are at once led to enquire how much of these schists must be of 

 Palaeozoic age. But even if we claim that the boundaries of the 

 group are everywhere actual lines of dislocation, we have to admit 

 that the thrust-planes and metamorphism which have affected it, and 

 which cannot easily be separated from those of the general mass of 

 the Highlands, must be at least of post- Arenig age. And in that 

 case we are naturally driven to ask how far the tectonic arrangement 

 and crystalline condition of the Highland rocks have been determined 

 or modified by movements that must be assigned to some date 

 between the period of the Arenig Group and that of the Lower Old 

 Ked Sandstone. Having had opportunities of studying the rocks 

 on the ground with each of the members of the Survey — 

 Mr. Gunn, Mr. Clough, and the Author of this paper — who had 

 surveyed the several areas in which they are developed, Sir Archibald 

 could bear witness to the patience and skill with which the mapping 

 had been carried out. If perfect agreement had not been reached 

 in the interpretation of the ])henomena, this difference of opinion 

 only served to indicate the difficulty of the problems and the diver- 

 sity of form under which they present themselves in the various 

 districts. The speaker was inclined to believe that the key to the 

 solution of these problems was most likely to be found in the North 

 of Ireland, where, in the County of Tyrone, rocks which, there 

 could hardly be any doubt, are a continuation of the Silurian Series 

 wedged in along the Scottish Highland Border, expand over a broad 

 stretch of country, and have been laid open in many natural and 

 artificial exposures. They include sedimentary rocks among which 

 are cherts and jaspers like those of the Arenig Group, and also 

 red shales and pale grits and limestones, which may represent the 

 Author's Margie Series. But their most remarkable feature is 

 the great development of their igneous masses, which comprise 

 ellipsoidal lavas, fine tuffs, and coarse agglomerates, together with 

 intrusive sheets and bosses. These rocks require to be carefully 

 mapped in detail, and their boundaries with the Londonderry Schists 

 on the one side, and the Palaeozoic formations on the other, must be 

 closely traced. This task will not only tend to clear up an obscure 

 chapter in Irish geology, but may help to remove some of the 

 difficulties which still stand in the way of a satisfactory interpreta- 

 tion of the geological history of the structure and metamorphism of 

 the Highlands of Scotland. 



Mr. Marr, being interested in the relationship of the schists of 

 the Highlands to the Palaeozoic sediments, wished to call attention 

 to certain slates which the late Hugh Miller, in his 'Cruise of the 

 Betsy ' 7th ed. (1869), p. 257, stated, on the authority of Dr. Emslie, 

 were graptolite-bearing. The late Prof. Nicholson had made a 

 cursory examination of these slates, and judged that they distinctly 

 possessed the characters of graptolite-bearing slates. He (the 

 speaker) urged that these slates should be re-examined. 



