Geological Society. 513 



bors of the old red sandstone ; and 3rdly, the anticlinal chain of 

 the Sidlaw Hills, consisting of the inferior or grey beds of the old 

 red sandstone, usually accompanied by trap. He further states, 

 that it represents, on a small scale, both geologically and physically, 

 the portion of Switzerland where erratic blocks are most abundant, 

 tlie Granii)ians with their crystalline rocks being comjjarable to the 

 Alps, the secondary chain of the Sidlaw Hills to the Jura, and 

 Strathmore to the great valley of Switzerland ; and that the resem- 

 blance is increased by the occurrence in Strathmore and on the 

 Sidlaw Hills of angular and rounded blocks of Gram])ian rocks. 



The superficial detritus of Forfarshire, Mr. Lyell divides into 

 three deposits : 1st, the thin unstratified covering on the Grampians, 

 derived from the disintegration of the subjacent strata, with a slight 

 intermixture of pebbles traceable to rocks at a higher level, not far 

 distant ; 2ndly, the unstratified materials enclosing boulders which 

 occur at the base of the hills on both sides of every glen, and not 

 due to taluses formed by landslips, but constituting terraces of 

 transported debris, with a nearly flat top, and sometimes with two 

 steep sides, one towards the river, and the other of less height to- 

 wards the mountain ; and 3rdly, the stratified gravels, sands and 

 clays which overlie the unstratified detritus. Mr. Lyell confines 

 liis observations jjrincipally to the second and third divisions. 



The terraces or lateral mounds very generally increase in width 

 and depth as thej^ descend from the higher to the lower glens, attain- 

 ing in the latter sometimes a thickness of 100 feet, and occa- 

 sionally so great a breadth as to leave only sufficient room for the 

 river to pass. The inferior part is always unstratified, consisting of 

 mud and sand, in which large angular and rounded fragments of 

 rocks are imbedded. These boulders are more and more rounded 

 as their distance increases from the hills whence they could have 

 been detached ; but they are more frequently flat-sided than pebbles 

 which have been rounded by water ; and they become more diversi- 

 fied in character by the junction of every tributary glen. In the 

 vipper jjart the mounds often consist of 40 to 80 feet of the same 

 materials as the lower, but regularly stratified. Mr. Lyell then 

 proceeds to illustrate his subject by describing in detail the pha?- 

 noraena presented by the valley of the South Esk and those of its 

 tributaries. 



The South Esk springs from a shallow lake nearly 3000 feet 

 above the level of the sea, and twenty miles from Strathmore. For 

 six miles the river flows through a district composed partly of gneiss, 

 traversed by veins of granite or eurite, and partly of granite. The 

 fragments derived from this high region may be traced downwards 

 continuously for twelve miles to Cortachie ; and as a proof that the 

 detritus forming the lateral mounds has followed the same down- 

 ward course, Mr. Lyell states that it jjreserves throughout, as well 

 in the main as in the lateral glens, an uniformly grey colour ; while 

 the detritus of the lower zone of mica-slate is invariably tinged red, 

 this colour being also imparted to the debris of the still lower por- 

 tions of the glens, notwithstanding the intermixture of pale brown 



Ann. ^ Mag. N. Hist. Vol. vii. 2 L 



