110 bulletin: museum of comparative zoology. 



that the composition of this rock throughout Jamaica is not always that 

 of quartz, mica, and feldspar, which are the constituents of true granite, 

 but more frequently feldspar, hornblende, and quartz, . . . and through- 

 out the whole series feldspar and hornblende occur more abundantly 

 than quartz." 



According to the microscopic studies of Cross, the exact nature of 

 these so called " granites," " syenites," and " porphyries " is as follows : — 



" Specimen No. 95. — On road to Blue Mountain Peak. A very dis- 

 tinct porphyry of the structural type common in the laccolithic masses 

 and intrusive sheets of probable early Tertiary age in Colorado, and 

 adjacent territory. The rock is near the line between granite-porphyry 

 and quartz-diorite-porphyry, its feldspar crystals are so much decomposed 

 that their character cannot always be made out. Some are plagioclase, 

 some may be orthoclase. I think the rock had hornblende crystals in 

 its fresh state. Now the ferro-magnesian constituents are so far de- 

 composed tliat they cannot be identified. They were never abundant. 

 The groundmass consists largely of quartz and orthoclase in a fine 

 earthy granular structure," 



" Specimen No. 146. — Feldspar Porphyry. District of Port Royal. 

 Salt Ptiver. Shelf 6, Institute of Jamaica, Labelled jSIetamorphic 

 Series. A much altered porphyry with small phenocrysts of orthoclase 

 and plagioclase and a dark silicate now indicated by calcite and chlorite. 

 The outlines of this latter constituent are not sufficiently clear to permit 

 inferences as to whether it was augite or hornblende. Groundmass has 

 some quartz but is mainly orthoclase feldspar." 



" Specimen No. 207. — Copper in syenite. District of Metcalfe, Parish 

 of St. Mary. Shelf 7, Institute of Jamaica, ' Granitic Series.' A granite 

 porphyry with much quartz in its irregularly granular groundmass. 

 Pink orthoclase feldspar is the principal component, in irregular large 

 grains seldom of good crystal form, and in the groundmass with quartz. 

 Chlorite replaces a former dark constituent, — probably biotite. 



"This granitoid group of rocks, exposed by erosion at many places in 

 the Blue Mountain region, is best developed in a limited area embraced 

 in the district of Port Royal (St. Andrew Parish, St. George, Portland, 

 Metcalfe, St. Mary), and immediately adjacent to the corner of the 

 parishes of St. Andrew, St. Mary, and St. Catherine. All these locali- 

 ties are in a small district of the island near the western half of the 

 Blue Mountain range, occurring in some places over considerable areas. 

 Sometimes these apparently occur beneath all the sedimentary rocks, 

 but usually they can be clearly seen as dikes, stringers, or masses pro- 



