THE GEOLOGY OF BERMUDA. 11 



Bermuda was sliowu 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 

 charaeters 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 higli 

 wind can, however, sweep along pieces of shell and coral larger and 

 heavier than one would at lirst suppose. The flat, thin valves of lamel- 

 libranchs are more likely to occur in drift-rock than shells rf 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 Si)ondylus weighing 1.8 grammes; 

 a valve of Chama, incrusted with tubes of Serpula, weighing 2.7 

 grammes; and a fragment of the coral iNIycedium, 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 Livoua 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.* AVhile the presence of marine fossils in a sand-rock 

 is an indication that it is a beach-rock, the drift-rock is quite apt to con- 

 tain the shells of land snails. The i)resence 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 uianifest 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 x)rocess of formation. At the 



*0i>. cit., p. 112. 



