354 
THE CRETACEOUS ROCKS OF BRITAN 
North Dorset there is often only a single thin layer of marl, and in 
one locality at least the Melbourn Rock actually passes down into 
a marly chalk of ordinary type (see p. 108). In West Dorset, again, 
the marl bed is often either thin or absent, though in other places 
it is well developed. 
On the other hand, there are places, as in East Dorset and 
in Sussex, where the marl swells out to a thickness of 12 or 14 feet. 
Another peculiarity of the horizon is that over a large area, from 
Berkshire to the border of Suffolk, its usual aspect is that of two 
thin layers of laminated marl with a bed of compact pure white 
chalk between them. 
It is not very easy to understand what could cause such a sudden 
change in the conditions of deposition over such a large area, but 
it is clear that some special occurrence took place or some geographi¬ 
cal change of more than local importance in consequence of which 
currents of considerable power were set in action, but it is evident 
also that these conditions did not last long, that they were of the 
nature of an interruption and could not be connected with the 
progress of a gradual and continued subsidence. 
Current action is plainly indicated by the laminated character 
of the marls, by the occasional occurrence of brecciated beds, and 
by the actual erosion of underlying chalk which seems to have 
occurred in some places (see p. 179). The Midland facies, moreover, 
shows two distinct episodes of current action with an intervening 
period of quiet deposition. Lastly, we have a suggestive fact in the 
occasional occurrence of small pebbles of quartz and quartzite, two of 
which have been found in Cambridgeshire and one in Lincolnshire. 
The most simple explanation of the facts which occurs to me is 
the supposition that there were two occasions or seasons of un¬ 
usually heavy rainfall, causing great floods in the valleys of the larger 
rivers which discharged themselves into the Cenomanian sea; the 
volume and velocity of these rivers being thereby so greatly in¬ 
creased that their currents carried muddy sediment far out to sea, 
just as the Amazon and other large rivers do at the present time; 
only that what is usual and continuous in the case of the Amazon 
was an unusual and exceptional occurrence in the case of the rivers 
which opened into the Cenomanian sea. 
The above hypothesis has the merit of invoking a natural cause, 
without the necessity of supposing any interruption to or reversal 
of the general movement of subsidence which is indicated by the 
nature of the preceding and the succeeding deposits. It will account 
for the sudden transition from nearly pure chalk to argillaceous 
marl, for the small thickness of the marly layers and for their epi- 
sodal mode of occurrence ; it will account for the local variations 
in the development of the marly beds and at the same time for the 
wide distribution of the argillaceous material. It will also explain 
the presence of the pebbles which are so likely to have been trans¬ 
ported in the roots of trees and plants, many of which must have 
been carried out to sea under the circumstances above suggested. 
