58 



BULLETIN : MUSEUM OF COMPAllATIVE ZOOLOGY. 



In places the Richmond beds grade up into irregular alternations of 

 impure clay, marls, and yellow limestones, which, in general, occupy a 

 transitional position between the obviously land derived beds of the 

 Blue Mountain Series and the ocean derived limestones of the Oceanic 

 Series. Argillaceous calcareous marls appear in the upper part of the 

 Richmond beds, become successively more and more frequent, and 

 finally dominate. These are accompanied by thin beds of impure blue 

 limestone of a segregational character, oxidizing yellow on weathering 

 and alternating with the marls which gradually increase in thickness 

 and relative proportion until they preponderate. Finall}', these yellow 

 limestones become more purely calcareous in ascending series as the 

 sediments become clearer and freer from land derived material, until 

 they finally pass into the purer White limestones. 



This formation has limited exposures at many places in the island, 

 notably around the lower margins of the interior basins in Trelawney, 

 Westmoreland, and Clarendon, and in the eastern parishes, as men- 

 tioned later. We shall first describe their occurrence in two typical 

 localities at Catadupa near Cambridge, and at Chapelton, Clarendon 

 Parish, respectively. Owing to certain differences which at present do 

 not permit of perfect correlation, these will be respectively termed the 

 Catadupa and Chapelton beds of the Cambridge Formation. 



The Catadupa Beds. 



They are well exposed on the east margin of Great River valley, in 

 the new cuttings along the Montego Bay Railroad between Ipswich and 

 Montpelier stations, and especially between points two or three miles 

 south of Catadupa and one mile north of Cambridge. Here the railway 

 cuttings reveal splendid exposures, and afford good places for collecting 

 fossils and studying the stratigraphy. These beds occur in a series of 

 short open folds, as shown in Figure 20. 



These folds are all less than two hundred yards in length, but the 

 continuity of the beds is so broken that their exact sequence and thick- 

 ness can be made out only with difficulty. Here the beds consist of 

 alternations of massive and friable strata of yellow blue limestone, one 

 to three feet in thickness, separated by thin bands of blue-black shale 

 containing oysters, large Cerithii, Lucina, Rudistes, Carolia, Triloculina, 

 etc. 



A general geological section of the east slope of the valley of Great 

 River, between Cambridge and Catadupa, showing the relations of the 

 formation, is as follows. 



