hill: geology of Jamaica. 79 



as the Cobre formation. The slopes are usually covered with dense vege- 

 tation, good rock exposures not being plentiful except in the immediate 

 streamway. Even then they are encrusted with tufaceous material, so 

 that their structure and arrangement are largely concealed. The follow- 

 ing extract from the notes of our traverse of this canyon will give an 

 idea of the composition and structure of these beds. Miles read north 

 from Spanishtovvn. 



0-4^ miles.^ — The river flows in a V-shaped alluvial plain indenting 

 the canyon to this point. 



4-^ miles. — A road metal quarry reveals the texture of the white lime- 

 stone, which is here composed almost entirely of small angular lumps of 

 firm limestone, chalky white in color, associated with a matrix of very 

 fine pulverulent chalky sinter, free from clay, but commonly termed 

 " White Marl/* as are all other soft formations. There are also a few 

 small lumps of compact gray or dove colored limestone (dolomite?). 

 This formation is of cavernous or " honeycombed " texture and weathers 

 into irregularly crystalline limestone, constituting ragged karrenfelder on 

 horizontal surfaces. Furthermore, the faces of the bluffs are in places 

 coated with self-derived tufa, and are so indurated that these outcrops 

 have a massive non-stratified appearance. This limestone differs from 

 that of the Cambridge and Montpelier beds in the entire absence of 

 lamination or traceable lines of bedding. 



4|^-8^ miles. — The limestone is continuously exposed in the vertical 

 bluff's through which the river cuts its way. No trace of bedding can 

 anywhere be made out, and the surfaces of the cliffs are everywhere 

 very indurated. 



8 miles. — The inclination of the beds, hitherto indeterminate, shows 

 a strong south dip. 



8^-9 miles. — The vertical cliffs show dips to south of 30° (estimated), 

 the angle increasing in steepness as we go north. While these exposures 

 are more massive looking than those before noted, quarries reveal the 

 same texture as that noted at Mile Post 4^. 



9-11 miles. Bog Walk Village. — Here the canyon ceases, and the 

 country, opens out into the interior valley of St. Thomas-in-the-Yale. 

 Superb /exposures of the Cobre limestone and its basal conditions are 

 shown lu the bluffs at the upper end of the canyon, as illustrated in 

 Figure 25. 



The pebble and buff-colored marls grade into the limestone, and repre- 

 sent its initiatory littoral. A few poor and indeterminate casts of fossil 



1 From Spanishtown. 



