306 James Durham — Post-Glacial Beds at Dundee. 



which the basal thrust-plane must make with the surface at its 

 outcrop. 



It is not, however, to be expected that the angle thus deduced 

 from the form of the mountain chain will agree with the dip of the 

 boundary fault along its foot ; for in a modern mountain chain the 

 actual base will seldom be exposed, and the faults which are visible 

 at the surface probably bear the same relation to the main thrust- 

 plane that the minor thrusts in the North- West Highlands bear to 

 the major thrust-planes of that region. It is only when the 

 mountain chain has been dissected to its very base that we can 

 hope to see the surface on which the main movement occurred. 



If I were to attempt, on this view, an ideal representation of the 

 Himalayas, I should draw, some little distance below Middlemiss's 

 Section VI, a thrust-plane making an angle of 14° with the 

 horizontal. The resemblance of the structure of the chain to 

 that of the North- West Highlands would then be conspicuous. 

 Most of the faults which are visible at the surface would appear 

 as minor thrusts, but it is possible that the great thrust-plane which 

 brings the crystalline schists upon the later beds was at one time 

 the basal plane of the range. 



I have no desire to argue that all mountain ranges lie upon the 

 outcrops of thrust-planes, or that there is necessarily a thrust-plane 

 wherever Professor Sollas has drawn one of his small circles. But 

 it remains certain that if a thrust-plane is a true plane, its outcrop 

 on the globe must be an arc of a circle, and that some mountain 

 chains are defined along their convex margins by thrust-planes. 

 Here, then, we seem to have an explanation of some of Professor 

 SoUas's small circles. 



Postscript. — After the proofs of this short note had left my 

 hands it occurred to me that the Calcutta earthquake of 1897 might 

 throw some light on the existence of the basal thrust-plane which 

 I have imagined in the Himalayas. It appears from Mr. Oldham's 

 valuable memoir on the subject that the earthquake originated in 

 Assam and not in the Himalayas themselves ; but it is interesting 

 to find that Mr. Oldham compares the structure of the Assam Hills 

 with that of the North- West Highlands of Scotland, and believes 

 that the earthquake was caused by a movement along a major thrust- 

 plane of low dip, accompanied by displacements along the more 

 steeply inclined reversed faults which reach the surface. 



V. — Post-Glacial Beds at Dundee. 

 By James Durham, F.E.S.E., F.G.S. 



IN the recently published volume of the Memoirs of the Geo- 

 logical Survey of Scotland, "The Geology of East Fife," by 

 Sir Archibald Geikie, it is argued that since the time when the 

 100 feet terrace was under the level of the sea there has been no 

 depression of the land in the Firth of Tay, only "a continuous 

 chronicle of gradual, if intermittent, uprise" (p. 321); also that 



