HILL: GEOLOGY OF JAMAICA. 43 
and sedimentation from coarse boulders and tuffs to finely triturated 
impure elay 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 Ridge. They 
undoubtedly underlie the surface rocks of the rest of tho 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-motamorphosed 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, which were described in a con- 
flicting manner,® chiefly as the “ Metamorphosed Series,” “ Black Shale,” 
1 Wall and Duncan, Quart. Jour. Geol. Soc. London, Vol. XXI., 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 in the 
Jamaican Report for beds included in our Blue Mountain Series :— 
Lower Tertiary or Conglomerate Series; the Trappean Series; sandstone and 
Sravel; upper sandstone clays and shales; black shale; Carbonaceous shales; 
Sandstone formation; conglomerate. 
Upper conglomerate ; upper conglomerate series ; lower conglomerate, ete. 
Purple shale and conglomerate ; purple shale and conglomerate formation. 
Cretaceous group; Cretaceous limestone; Cretaceous series; Cretaceous and 
Hippurite 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, 
Branite, and syenite of Tertiary age. 
