10 THE GEOLOGY OF BERMUDA. 



shore the calcareous sands resulting from the disintegration of shells 

 and corals, precisely as in other places the waves sweep up the silicious 

 sands of an ordinary beach. The beach sand-rock is therefore formed 

 chiefly between the levels of low and high tide, though the action of 

 storms may cause it to extend somewhat above the ordinary high-tide 

 level. The drift sand-rock is formed by the action of winds seizing the 

 dry sand at the upper margin of the beach and transporting it further 

 inland and to greater elevations. The drift sand-rock may therefore be 

 formed at any level, from that of high-tide upward. The cement which 

 converts all these fragmental deposits into solid rock is formed by the 

 solution of the calcareous particles themselves. 



A most important step in the investigation of the history of a coral 

 island is the recognition of the respective distribution of these three 

 kinds of rock. The discrimination of the true reef-rock from the sand- 

 rocks is not usually a difficult task. The reef-rock, whether fossilifer- 

 ous or not, is usually readily distinguishable by the impalpable com- 

 pactness of parts of the mass, resulting from the consolidation of the 

 finely triturated coral mud; while the sand -rocks, even when appearing 

 quite compact, will almost invariabl}^ reveal on closer examination their 

 arenaceous texture. 



The discrimination of the two kinds of sand-rock from each other is 

 much more difficult. Indeed, no absolute criteria exist for the discrimi- 

 nation of beach-rock and drift-rock, though serviceable indications may 

 be obtained from the texture, lamination, and fossil contents of the 

 rocks. The beach-rock is, on the average, of coarser grain than the 

 drift- rock, as the wind sweeps along chiefly the finer sands; but some 

 specimens of the drift-rock are coarser than some specimens of the 

 beach-rock. The beach-rock is, on the average, more perfectly consoli- 

 dated than the drift-rock, but in this character also both rocks vary 

 widely. Drift-rock, when submerged by a subsidence subsequent to 

 its deposition, may come to assume the degree of consolidation usually 

 observed in beach-rock. On the south shore of the Main Island, near 

 Spanish Rock, I observed strata perfectly continuous dipping towards 

 the water, exceedingly hard at the margin of the water, but becoming 

 considerably softer as they were traced upward and landward. Mr. 

 Ebenezer Bell, who some years ago had charge of some worlcs in prog- 

 ress on Boaz Island, informed me that he found that rock so soft as to 

 crumble in one's fingers became quite hard on immersion for a week or 

 a fortnight in sea- water. Some of the hardest rock which I observed in 



