HUGHES : INGLEBOROUGH. 
361 
before. In one place you find a great number of one form, in 
another adjoining area the fossils numerically characteristic are 
different. 
Let us for a moment consider the conditions under which 
basement beds are formed. If a limestone is being wasted away, 
by various denuding agents, and thus furnishes the material for 
a new series of deposits, we may have in the case of rapid accu- 
mulation a limestone breccia, or, under slower action, may have 
a conglomerate, or, perhaps, only tlie insoluble residuum as a 
red clay or loam. Or if, as in the case of the Upper Chalk, it 
contains flints, we may have a bed of flint pebbles, as in so many 
of our Tertiary rocks, or a bed of subangular gravel. 
The occurrence, therefore, of fragments of the harder parts 
of the underlying rocks, sporadically or in patches, over any 
area, points to the lapse of time to allow for the upheaval of the 
older rocks and for their subsequent denudation and the carrying 
away and sorting of the material over adjoining areas. 
But the division between the newer and older series is 
not always easy to trace, because there is not everywhere a 
conspicuous conglomerate, and the newer mud is very like the 
older from which it was derived. That is the difficulty we have 
to face in tracing the boundary between the Silurian and the 
Bala Beds. But still with care it can be done. 
If the older beds have been violently disturbed before the 
deposition of the newer series of which we are examining the base, 
there is seldom any difficulty in tracing the basement bed. It 
is transgressive across the upturned edges of the older rocks, 
and as it is made up of fragments of the different strata of which 
they are composed, we can sometimes infer the direction in 
which the material has travelled, and even find evidence as to 
the mode of transport. We can measure the amount of rock 
removed from the ancient surface before the deposition of the 
newer beds, and get a measure of the interval that elapsed be- 
tween the commencement of the uplift and consequent destruc- 
tion of the older beds, and the deposition of the basement bed 
of the newer. The greater this lapse of time the greater as a 
rule is the change in the forms of life brought about by the 
change of conditions. 
