906 PROFESSOR J. PRESTWICK OK THE EVIDENCES OF A SUBMERGENCE 
entire bones, as well as by their freedom from all traces of gnawing; conditions in 
marked contrast with those presented by the bones of the caves, which have 
commonly heen gnawed by predaceous animals, and with those of the fluviatile 
deposits which are usually more or less worn. 
The physical characters of this drift are equally well marked. The detritus of 
which it is composed, is always of local origin and angular. The component 
fragments retain their sharp angles uninjured. It is without stratification, and 
yet not without a certain order such as might result from successive masses of 
debris sliding down slopes and shot into hollows. In many respects, such as its wide 
distribution, and its want of stratification, it resembles a subaerial glacial detritus, 
but its entire want of wear, the presence of and condition of the bones, and especially 
the presence of delicate land shells, are incompatible with such an origin. The 
absence of foreign transported materials or boulders, and of marine shells, is equally 
incompatible with a submarine glacial origin. 
Taking all these facts into consideration, the only agent which appears to me 
capable to have produced such results, is that of an upheaval of a submerged land 
following upon a wide-spread submergence. This upheaval by displacing the super¬ 
incumbent body of water, would give rise, as shown in a paper by the late Mr. W. 
HorKiNS, of Cambridge,^' to divergent etfiuent currents, which would sweep down 
from the higher to the lower levels the debris of the submerged surface. Such would 
have happened, if after a temporary submergence the land had again been upheaved, 
and the former levels approximately restored. All the phenomena presented by this 
Rubble-drift are explicable upon this hypothesis, and upon none other that I can see. 
Of the submergence we can only infer that it was slow and gradual, but of the 
upheaval we have evidence to show that it was by stages alternately slow and more 
or less rapid. That evidence is best exhibited, not in districts where the rocks are of 
an uniform character, as then, in the absence of any difference in the debris, the 
rubble has the appearance of a talus without divisional planes, but where the rocks 
are composed of beds of different textures and degrees of hardness then the debris 
shows sorting and a kind of rough bedding, and this gives a clue to the manner in 
which the accumulation took place. These conditions obtain best in chalk districts, 
* ‘ Quart. Journ. Geol. Soo.,’ vol. 4, p. 90, Mr. Hopkins, sliows Hat if a considerable area at the 
bottom of the sea were suddenly elevated, currents, the velocity of which would depend principally 
upon the depth of the sea, would diverge in all directions from the central disturbance.- “ Calculations,” 
he says, “ prove beyond all doubt that paroxysmal elevations, beneath the sea, varying from 50 to 
100 feet in height, may produce currents of Avhich the velocities shall vary from at least 5 or 6 to 15 
or 20 miles an hour, provided the depth of the sea do not exceed 800 or 1000 feet.” In considering 
the magnitude of the blocks which might be moved, he found that the force exerted on a surface of 
given magnitude increases as the square of the velocity, and that it “ varies as the sixth power of the 
velocity of the current^' But the movements must be repeated for large blocks to travel beyond short 
distances. In this inquiry, however, we have to deal with smaller quantities and less paroxysmal 
jiiovements. 
