178 Mr. De la Beche on the Geology of Jamaica. 



Proceeding with the continuation of this formation into St. Thomas-in-the- 

 Vale, we find the lower marls_, which are sometimes schistosej in the vicinity 

 of Pimento Grove^ and on the southern side of the trap in that district : from 

 hence to Rio Magno Pen the limestone will be observed in great abundance. 



The hills on either side of Williamsfield and Sandy Gully estates, are 

 formed of compact white limestone resting upon trap rocks ; the strata, as 

 before remarked, appearing to have been once continuous, and the whole 

 looking like a valley of denudation. 



The mountains that surround the great basin of St. Thomas-in-the-Vale 

 are almost all composed of the white limestone formation ; and the basin itself 

 would seem convertible into a large lake, if the ravine through which the Rio 

 Cobre flows, called the Walks' Road, between the Bog Walk and Spanish 

 Town, were stopped up. 



Upon Mount Olive estate in this parish there is a singular natural bridge, 

 formed of compact white limestone. A small river, flowing in a precipitous 

 ravine, is crossed by a natural arch, beneath which it runs (see Plate XX.). 

 This natural bridge is lofty, and of sufficient breadth at the top to permit a 

 cart-road over it, which has accordingly been established. If it were not for 

 the arch, the water would be blocked up by this natural wall or barrier, the 

 cliffs rising high and precipitous on either side, and preventing its escape la- 

 terally. The cavernous nature of this limestone is curiously shown on the 

 same property, by the manner in which the water is procured to turn the 

 water-wheel. A rivulet runs on the side of the hifl opposite to that on which 

 the works are placed ; and to obtain water for the mill, the rivulet is dammed 

 up at a given place, when it finds its way through the hill, rushes out of a 

 hole near the works, and turns the wheel. 



The Walks' Road ravine exposes a good section of compact white limestone 

 and white marl beds ; the former are seen varying in thickness, and dipping 

 to the S.W. or S.S.W., near the opening of the defile into the basin of St. 

 Tliomas-in-the-Vale. 



The formation under consideration forms the mountains on the north of the 

 diluvial and alluvial plain of St. Catherine's, and constitutes small hills rising 

 out of it, close to Spanish Town, and on the road to Kingston. A white 

 chalky marl is exposed on the same road between the former place and the 

 Ferry. The Healthshire Hills, and the hill that rises above Port Henderson 

 (opposite Port Royal) are also composed of the same rocks. 



The mountains which join the elevated land of Stony Hill with those of 

 St. Catherine's, and which bound the plain of Liguanea for a considerable 

 distance on the north, belong to this formation. On the southern ascent to 



