THE GEOLOGY OF BERMUDA. sk 
Bermuda was shown by other characters to be unmistakably drift-rock 
A more reliable distinction is found in the lamination, the beach-rock 
showing a gentle and tolerably uniform dip towards the water, while 
the drift-rock shows the high and extremely irregular dips which are 
characteristic of wind-blown sands. But not every section exhibits 
characters sufficiently marked to settle the nature of the rock, since the 
beach-structure admits of a considerable degree of irregularity in dip, 
while wind-blown sands in a long ridge or dune may have for long dis- 
tances a gentle and nearly uniform dip. The indication furnished by the 
fossil contents of the rocks is important. The beach-rock is often richly 
fossiliferous, containing shells and pieces of coral of considerable size. 
The drift-rock can, of course, ordinarily contain no relics of marine ani- 
mals except fragments so small as to be blown by the wind. A high 
wind can, however, sweep along pieces of shell and coral larger and 
heavier than one would at first suppose. The flat, thin valves of lamel- 
libranchs are more likely to occur in drift-rock than shells cf gastero- 
pods. In the recent sand-drifts at Tucker’s Town I collected a number 
of organic relics, thinking they might afford some indication as to the 
limit of size of marine fossils likely to occur in the drift-rock. Among 
them were a fragment of the shell of Spondylus weighing 1.8 grammes; 
a valve of Chama, incrusted with tubes of Serpula, weighing 2.7 
grammes; and a fragment of the coral Mycedium, having a length of 45 
millimeters and a breadth of 30 millimeters, and weighing 8.3 grammes. 
Of most frequent occurrence in the drift-rock of Bermuda is the large 
and heavy shell of Livona pica. This seems at first sight utterly para- 
doxical, as the shell is altogether too large to be moved by the wind. 
The true explanation is undoubtedly that given by Nelson, who states 
that he has on more than one occasion seen soldier-crabs running about 
in these shells.* While the presence of marine fossils in a sand-rock 
is an indication that it is a beach-rock, the drift-rock 1s quite apt to con- 
tain the shells of land snails. The presence of snail-shells cannot, how- 
ever, be regarded as a sure proof of drift-rock, since they might easily 
be washed down by rains from a bank or bluff above the beach, and 
imbedded in the beach sands. 
That there can be no absolute distinction between beach-rock and 
drift-rock will be manifest from the consideration that the two forma- 
tions are in their origin strictly continuous. Near Elbow Bay and at 
Tucker’s Town, sand-hills are now in process of formation. At the 
* Op. cit., p. 112. 
