260 PLANTS, SEEDS, AND CURRENTS 



extensive low sandy tracts, overlying old reef-rock, belong to the 

 present, the " nuclei " of seolian sandstone, around which they have 

 been formed, belong to the past, a past when probably almost the 

 whole of the Turks Bank was occupied by land of seolian formation, 

 four-fifths of which have disappeared at the hands of the marine 

 and aerial eroding agencies. 



All the ten islands or cays are composed either entirely or in part 

 of the seolian sandstone, the growth in size in later times by the 

 development of low land around " nuclei " of the seolian rock being 

 chiefly characteristic of the larger islands. 



The Molian Sandstone. — This is a white calcareous sandstone of 

 the Bahamian oolitic type. Whilst on the exposed surface the 

 materials are often cemented into a hard crust, which is some 

 inches thick and possesses an almost flinty fracture, they are usually 

 loosely compacted beneath, so that the rock readily breaks down 

 between the fingers. The crust, however, is generally broken up 

 into slabs, as explained below, thus exposing the less consolidated 

 rock which, readily disintegrating, has furnished much of the loose 

 sand that occurs on the surface of the interior of the larger islands. 

 It is as a semi- compacted sandstone that the rock typically presents 

 itself ; but in the smaller low flat cays it is the hard crust that attracts 

 most notice, and one might imagine that one was dealing in the 

 mass with a hard, honeycombed limestone, if the attention was 

 confined only to the surface characters. It is in the face of the line 

 of bluffs on the east side of Grand Turk and in the coast cliffs of the 

 other large cays that this formation is best exposed; but one may 

 find good exposures at times in the smallest of the islands when they 

 possess any elevation, as on Round Cay. 



This formation sometimes offers itself in thick beds of loosely 

 compacted sandstone composed of usually well-rounded grains 

 varying from 0*5 to 1*5 mm. in size. At other times the rock is made 

 of finer materials, between 0-3 and 0-5 mm. in size and rather more 

 consolidated. Such finer sandstones often display a laminated 

 structure, the layers being usually horizontal; but they are occa- 

 sionally inclined, in which case cross-bedding may at times be 

 observed. The hard crust exhibits scattered grains in a compact 

 matrix, and has the appearance of a consolidated calcareous ooze; 

 but, as is shown below, it has a very different history. All these 

 rocks dissolve readily in acid, leaving scarcely any residue. 



Nowhere did I find any marine shells or other marine remains in 

 the aeolian rocks. However, on the sandy slopes of Gibb Cay I 

 found gathered together on a ledge great numbers of old Bulimoid 

 shells of a large species of land-mollusc. Whether they had been 

 freed by the disintegration of the sandstone from which the loose 

 sand was derived, or whether they represented a molluscan fauna 

 that had subsisted on vegetation which had been destroyed by the 

 goats, I could not determine. The first explanation seems to be 

 most probable, since A. Agassiz (p. 20) remarks that land-shells 

 similar to those now living on the islands occur in the seolian rocks 

 of the Bahamas. 



We come now to the consideration of the hard surface-crust of the 



