518 T. Mellaril Reade — Overthrusts. 



VI. — The Mechanics of Overthkusts. 

 By T, Mellard Rbade, C.E., F.G.S. 



IN attempts to unravel some of the weightier problems of geology- 

 it has lately been assumed that certain discordances of stratifica- 

 tion are due to the thrusting of old rocks over those of a later 

 geological age. Without in any way suggesting that the geology 

 has in any particular instance been misread, I should like to point 

 out the difficulties in accepting the explanation looked at from 

 a dynamical point of view when applied on a scale that seems to 

 ignore mechanical probabilities. Some of the enormous overthrusts 

 postulated are estimated at figures approaching 100 miles. Have 

 the authors considered that this means the movement of a solid 

 block of rock or rocks of unknown length and thickness 100 miles 

 over the underlying complex of newer rocks ? If such a movement 

 has ever taken place, would it not require an incalculable force to 

 thrust the upper block over the lower, even with a clean fractured 

 bed to move upon ? Assuming that the block to be moved is the 

 same length as the overthrust, the fracture -plane would in area be 

 100 X loo = 10,000 miles. I venture to think that no force applied 

 in any of the mechanical ways known to us in Nature would move 

 such a mass, be it ever so adjusted in thickness to the purpose, 

 even if supplemented with a lubricant generously applied to the 

 thrust-plane. These are the thoughts that naturally occur to me, 

 but as my mind is quite open to receive new ideas I shall be glad to 

 know in what way the reasoning can be met by other thinkers. 



E, E "V I E -W S. 



I. — Memoirs of the Geological Survey of England and "Wales. 



The Coals of South Wales, with special reference to the Origin 

 AND Distribution of Anthracite. By Aubrey Strahan, M.A., 

 Sc.D., F.R.S., F.G.S., and W. Pollard, M.A., D.Sc, F.G.S. ; 

 assisted by E. G. Radley. pp. v, 74, with 10 folding plates. 

 London : printed for H.M. Stationery Office, 1908. Price Is. &d. 



THIS memoir includes a chapter entitled "Historical and Intro- 

 ductory," which deals with previous knowledge of the occurrence 

 and distribution of anthracite in South Wales, and states certain facts 

 as to the distribution of anthracite and its passage into other forms of 

 coal. " The changes undergone by the coal present certain stages . . . 

 from House Coal, or the most bituminous, the change is gradual into 

 Steam Coal, and from Steam Coal into Anthracite." 



We are told that tbe " Anthracitic regions lie in the north-westeni 

 corner of the Carmarthenshire, Brecknock, and Glamorganshire field 

 and in Pembrokeshire," and "Lines of equal anthracitisation circle 

 round an area which extends from Kidwelly to Glyn Neath. In 

 Pembrokeshire all the coal is anthracitic." 



The second generalization states that " the seams all show the 

 change on approaching the anthracitic region, but the higher seams 



