On the Geology of the Key of Sombrero, W. I. 255 



dation may be understood, is given in Fig. 1, the beds being 

 lettered upwards from A to F for distinction. As only a rem- 

 nant of small area exists of the highest bed, there is only one of 

 the quarries which gives a section of the whole. From the 

 sketch of this section (Fig. 2) an idea may be obtained of its 

 general appearance and constitution. In this and all sections 

 of the southern level, as in the face of the surrounding cliff, 

 only two horizontal divisions are prominent ; but the effect of 

 the upper is heightened, upon the fresh surface of the sections, 

 by the contrast between the greyish band of D and the white 

 limestones above and below. 



Here, as usual in a geological problem, only a part of the 

 facts are furnished ; of the six leaves in the centre of the book 

 the very margins have been torn away. Doubtless the older 

 and larger portion of the formation is submerged below the 

 sea-level. The submarine face of the cliff, about eighty feet in 

 depth, must certainly consist of limestone, and it is probable 

 that the basis of the whole calcareous stratum will not be 

 reached before a depth of one or two hundred fathoms or more, 

 as off the edge of the bank. On the other hand, several of the 

 most recent beds may have been removed by denudation from 

 the summit of the Key. 



Concerning the former extent of all these beds we have no 

 data, but a clue exists in the area of the submarine bank. 



§ 6. Oscillations. — The marine deposits appear to have 

 formed upon the area, oscillating vertically, of the bottom of a 

 lagoon, more or less inclosed ; and even in the earlier deposits 

 of extraneous origin, succeeding the emergence of the area 

 from the sea, we may detect traces of the influence of the basin- 

 form peculiar to the surface of an elevated atoll. These oscil- 

 lations generally seem to have consisted of a prolonged subsi- 

 dence, during which each bed was formed, and of a sudden ele- 

 vation to a greater or less height, marked when it was greatest 

 by the sharp upper limit of the bed. It has thus happened that 

 the organic components of the bed are in some degree peculiarly 



