618 MR. F. A. STEART ON OVERTHRUSTS AND OTHER [ Nov. 1902, 
showed that a pair of trough-faults had let down a portion of the 
coalfield, and had given rise to appearances which had been inter- 
preted as due to an unconformity in the Coal-Measures. One of 
these faults was parallel to the south-western margin of the basin, 
and to the ‘ Horse, and if continued would pass close to the latter. 
The ‘ Horse’ was perhaps the best-known example of its kind in 
the British Isles. Its existence had been attributed to contempo- 
raneous fluviatile denudation, but Mr. Arnold Thomas, F.G.S., had 
informed the speaker that the side of that portion with which he 
was familiar, so far from presenting the appearance of a river-bank, 
showed a clean-cut slickensided surface: he had, moreover, proved 
the existence of coal—presumably the seam that was being worked 
at the time—beneath the level of the workings, and thought that 
the ‘ Horse * was probably due to a pair of trough-faults. 
Sir AncHIBALD GrIkrE remarked that the progress of investiga- 
tion in recent years had shown that overthrusts are of much more 
frequent occurrence than had previously been surmised. They 
occur even in districts which would not have been thought to have 
undergone any marked amount of disturbance. He had, lately 
found them by no means infrequent in the various subdivisions of 
the Carboniferous System in Fife. And they have been shown not 
to be confined to our older formations, but to be well marked even 
among the Cretaceous formations of the South of Engiand. He 
thought it quite possible that some structures, which had been 
otherwise interpreted, before the prevalence of overthrusts had 
been realized, might be explained by faulting of this kind; though 
there could be no doubt as to the existence of many proofs of contem- 
poraneous erosion, to which such an explanation would not apply. 
It was by such detailed observations and measurements as had been 
furnished by the Author of the paper that these disputed questions 
could best be settled. 
Mr. Srrawan thought that the Society was indebted to the 
Author for the care with which he had made observations under 
the difficulties inseparable from mining work. He agreed in the 
main with the Author’s conclusions. No one probably would deny 
the existence of true ‘ wash-outs’ in the Coal-Measures. All the 
phenomena of contemporaneous erosion had been observed in more 
than one district. He (the speaker) had himself seen repeatedly 
in a neighbouring district evidence of movement along bedding- 
planes, or along planes diverging but slightly from the bedding. 
Shale-partings between coals had been sharply puckered, or bands of 
coal rendered schistose, while the beds above and below remained 
unaffected. Open quarries showed overthrusts in coal-seams 
ranging from a few inches to upwards of, 20 yards, and the 
doubling or even trebling of the seams. In other places the coal 
had been squeezed out for several yards, leaving an area of dead 
ground, such as that described by the Author. He suggested that 
the tendency of the overthrust to decrease towards the surface 
might be due to its splitting into subsidiary planes of overthrusting 
along the coals of the Radstock Series. 
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