244> Mr. De la Beche on the Excavation of Valleys, 



cavities that pervade the white hmestone of Jamaica. One 

 great valley is remarkable ; it is situated between the Carpen- 

 ter and Santa Cruz mountains, and is excavated in a white 

 limestone interstratified with a red sandstone. It continues 

 inland some miles from the sea at Alligator Pond Bay. The 

 bottom is in general an arid plain or savanna, here and there 

 studded with insulated masses of rock bounded by broken cliffs ; 

 these rocks are covered with vegetation, and resemble, in this 

 respect, oases in a desert. They consist of white limestone in 

 nearly horizontal beds, varying from four to ten feet in thick- 

 ness, and seem to be the remains of continuous strata, which 

 have been nearly destroyed by some great force, but certainly 

 not by that of the waters that now run in the valleys; for there 

 is neither river nor rivulet throughout its whole extent. The 

 river that rises suddenly near the sea, and flows but a short di- 

 stance at the lower termination of this long and wide valley, is 

 most probably derived, like many similar Jamaica streams, from 

 waters swallowed by sink-holes in the interior of the island. 



III. — Valleys of Denudation siibsequently ctit into Ravi7ies, and 

 otherwise modifed hy existing Causes. 



As the smooth-sided valleys of denudation I have been de- 

 scribing form the present drain of pluvial waters, I proceed to 

 consider what changes these waters, and the streams resulting 

 from them, have effected in the original outline of such valleys. 



These changes are often very considerable, and sometimes 

 so modify the valleys that their features derived from de- 

 nudation are nearly obliterated. When the original valley has 

 been scooped out of soft substances, such as soft sandstone 

 or conglomerate, a river resulting from the drainage of the 

 land will have cut a gorge or ravine with cliffs of greater or 

 less height on either side according to circumstances. Of this 

 modification of a valley, the Vallon Obscur, near Nice, will 

 afford an example ; a, a, fig. 2, are the sides of the original 

 valley ; b, 5, the gorge or ravine formed by the torrent that has 

 cut through the nearly horizontal strata of tertiary sandstone 

 and conglomerate down to its present bed. The same rocks in 

 the same vicinity afford other examples of this modification 

 of original valleys, so that in some cases it would be difficult 

 to say whether they are original, or have been produced by 

 actual meteoric causes. These conglomerates and sandstones 

 are generally of easy disintegration, and readily give way. 



IV. — Action of Rivers in nearly level and spacious Valleys. 

 Rivers when flowing through extensive and nearly level 

 valleys seem to eflect little beyond an occasional change of 



bed; 



