HILL : GEOLOGY OF JAMAICA. 43 



and sedimentation from coarse boulders and tuffs to finely triturated 

 impure clay shale, — a process indicating extensive working over. These 

 rocks are the material of the Central Mountains, composing the emi- 

 nences above 3,000 feet, such as the Blue Mountain Kidge. They 

 undoubtedly underlie the surface rocks of the rest of the island as occa- 

 sionally revealed by erosion through the white limestone which veneers 

 them, as seen in some of the central basin valleys, the canyons of the 

 marginal streams, and certain bluffs of the back coast border along the 

 northwest coast. 



Nowhere on the island can all the beds of the series be seen in con- 

 tinuous exposure. As has been noted ^ concerning the rocks of the 

 Blue Mountain district, *' the strata are so excessively disturbed, so 

 traversed and semi-metamorphosed by dikes of syenite and mixed up 

 with porphyritic masses, that it is impossible to observe the intricacies 

 of the stratification or to determine the sequence of the beds inter se 

 without a lengthened and detailed investigation." Sufficient is known 

 to state that it probably exceeds 5,000 feet. 



Previous attempts to classify the rocks which are collectively arranged 

 in this series have been confusing and unsatisfactory. De la Beche,^ 

 who first described them in 1828, referred them as follows : " Submedial 

 or Transition Rocks " (Cambrian), " Submedial or Transition Lime- 

 stone " (Cambrian), Medial or Carboniferous Rocks," and the "Super- 

 medial or Secondary Rocks." The members of the later official survey 

 showed that De la Beche's age determinations were erroneous, and the 

 rocks were of Cretaceous and Eocene age. These writers gave no satis- 

 factory statement of the subdivisions, w^hich were described in a con- 

 flicting manner,^ chiefly as the *' Metamorphosed Series," "Black Shale," 



1 Wall and Duncan, Quart. Jour. Geol. Soc. London, Vol. XXL, 1865. 



2 Mem. Trans. Geol. Soc. London, No. 36, 1826, pp. 151-169. 



3 The following names, many of them no doubt synonyms, are used m the 

 Jamaican Report for beds included in our Blue Mountain Series: — 



Lower Tertiary or Conglomerate Series ; the Trappean Series ; sandstone and 

 gravel ; upper sandstone clays and shales ; black shale ; Carbonaceous shales ; 

 sandstone formation ; conglomerate. 



Upper conglomerate ; upper conglomerate series ; lower conglomerate, etc. 



Purple shale and conglomerate ; purple shale and conglomerate formation. 



Cretaceous group ; Cretaceous limestone ; Cretaceous series ; Cretaceous and 

 Htppurite limestone. 



Metamorphosed Series ; Metamorphosed or altered rocks ; Metamorphosed 

 Series and igneous dikes ; Metamorphosed conglomerate ; Altered stratified and 

 igneous rocks; Altered Stratified Series. 



Igneous formation ; igneous rocks ; porphyritic dikes, granite rocks, syenite, 

 granite, and syenite of Tertiary age. 



