12 THE GEOLOGY OF BERMUDA. 



former locality the advaucing dune has nearly buried two small houses, 

 and is encroaching upon cultivated grounds. Xelsou* gives us an 

 account of the beginning of this invasion, and Thomson t describes the 

 present condition of the dune. At these localities there is an oi)por- 

 tunity to study the mode of formation of the two kinds of rock, and 

 to observe the perfect continuity of the two formations. As the wind 

 catches the sand on the upper and drier part of the beach, and moves 

 it landward, the beach merges above, with no perceptible line of 

 demarkation, into the base of the sand-hills. But though, in the 

 nature of the case, there can be no absolute distinction between beach- 

 rock and drift-rock, it is practicable, by noting all the indications of 

 texture, lamination, and fossil contents, to decide in most cases with 

 confidence whether the rock exposed in a particular section is beach- 

 rock or drift-rock. 



There is no reef-rock in Bermuda in situ above the water-level. ^STel- 

 son speaks of blocks of coral reef imbedded in the rock on the south 

 shore of the Main Island.f I observed detached blocks of reef-rock on 

 the shore at Stock's Point, but none in situ. In the statement, "Toward 

 the shores the solid reef- rock outcrops," § Dana is apparently misled, in 

 a way very natural for one who has not visited the locality, by a state- 

 ment of Nelson. The rock described in Dana's quotation from Nelson 

 as "very hard, fine-grained or compact limestone, in which scarcely a 

 vestige of organic structure is to be seen,"i| shows on careful examina- 

 tion an arenaceous texture, though consolidated by percolating waters 

 to a sub-stalagmitic condition, exhibits traces of irregular lamination, 

 and contains fossil shells of laud snails. It is unquestionably an 

 extremely hard drift-rock, such as is found at several localities and at 

 various altitudes. 



Beach-rock occurs at various localities along the shore of the isl- 

 ands. Thomson's statement that the Bermuda limestone is entirely an 

 "uiEolian formation"^ is certainly inaccurate. I have never observed 

 the beach-rock in the interior, nor at an altitude of more than about 

 15 feet above the water-level. To the category of beach-rock may 

 undoubtedly be referred the fossiliferous stratum described by Nelson ** 

 as appearing in the chain of islands stretching across the mouth of 

 Crow-lane or Hamilton Harbor. This stratum reaches an elevation of 

 about G feet above the water, and its nearly horizontal lamination con- 



* Op. cit., pp. 109, 110. t Op. cit., Vol. I., pp. 289-291. 



t Op. cit., p. 111. $Dana, Corals and Coral Islauds, p. 220. 



II Nelson, op. cit., p. lOG. II Op. cit.. Vol. I., p. 287. *' Op. cit., p. 111. 



