HUGHES : EsGLEBOROUGH. 
289 
at " a " and try to trace the limestone ah. The bottom of the 
valle\^ is rising and the slope increasing from G to H ; the effect 
of this is to make us rise continually and take the path indicated 
by the dotted line af, so that finding indications of limestone 
at c and again at / we draw our boundary along af. I have 
known one of our very best field geologists led on to the second 
limestone above that on which he started by this source of 
error, which is a kind of optical delusion. 
To avoid it it is necessary on such ground always to carry 
an aneroid barometer, or better a pocket level, by which observa- 
tions can be taken on fixed objects in front with sufficient accuracy 
to prevent our travelling up when we think we are going down, 
or vice versa. 
Fig. 8. 
DIAGRAM EXPLAINING A SOURCE OF ERROR IN RUNNIN'G BOUNDARY LINES 
ON INGLEBOROUGH. 
There are some small faults on Ingleborough which are 
generally quite easy to detect and trace. Some of them seem 
to be cracks in the Carboniferous rocks recurring over pre- 
existing lines of disturbance in the underlying Silurian and 
Bala Beds. Some, occurring as they do on such a bluff as 
Ingleborough, justify our inquiring whether they may not be 
analogous to the " marginal effects " of earthquakes. 
But interesting as these are, and wide as may be the field 
of inquiry suggested by them, the irregularities of dip which 
are practically the most important for us on Ingleborough, are 
those sudden changes of direction and amount over areas of 
limited extent to which I have frequently had to refer. As, 
for instance, in the case of the converging dips round swallow 
holes where the rock is breaking down into the gradually eroded 
