288 
HUGHES : INGLEBOROUGH. 
their velocity is so great that they strike the opposite side of 
the chasm with such momentum that after a great flood the 
rock is seen to be bruised all over, teaching us that a great part 
of the subterranean denudation is due to the action of boulders 
and pebbles upon the rock, which is here battered by fragments 
bounding from side to side with ever increasing velocity 
in their long descent of over 300 feet. This was especially 
conspicuous after the great flood of 1872, and the pounding up 
of the black limestones was also curiously forced upon our notice 
in another way on that occasion, for the people down the valley 
at and below Clapham observed that a strong gas-like smell 
arose from the flood — I suspected its origin and, as the water 
fell, found that an enormous quantity of black sand had been 
brought down. On rubbing this between my two hands 1 found 
that it ga^^e off tlie same fetid smell which had been noticed. 
This was very remarkable, for, on that occasion, the water liad 
passed through the tarn, but the air tangled in that seething 
flood had not ti]ne to escape even there. We must, however, 
remember also that the volume of water was so great that it 
produced a considerable current even in tlie deep waters of the 
tarn, and perhaps some of the mud, with decomposing vegetable 
matter from dead leaves, &c., may have got stirred up and 
contributed its share of carburetted hydrogen to the tangled air. 
Cautions and Suggestions. 
We have often to run a very cold scent along the flanks 
of Ingleborough, and as a good deal depends upon our trusting 
to persistence of dip, thickness, &c., in tracing the various sub- 
divisions of the Yoredale rocks under the drift and peat bogs, 
a few words of caution bought by experience may be helpful. 
Wiien following a.n obscure boundary line among beds which 
are not easily distinguished from one another, as is ths case 
among the black limestones of Inglebcrough, it is very easy 
to drop or rise from one formation to another similar one in 
crossing a swampy tract of peat or drift, or a rough bank of 
talus. Supposing, for instance, we are going north along the 
east flank of Ingleborough (see Fig. 8), where the beds ah, cd, 
ef, are sloping almost imperceptibly to the north, and we start 
