HILL: GEOLOGY OF JAMAICA. 109 
large amount of silica, The rock is to be generally classed with the 
andesites rich in silica and poor in iron, and thus may be a dacite.” 
Whether these are exotic or have been derived from more ancient 
rocks composing tho structure of tho island and now concealed by later 
formations, are speculative questions. For the present it is only safe 
to say that, together with all the other pebbles of the Richmond beds, 
they are detritus derived from rocks which were originally made in 
Previous geologie epochs, and that there are no indications of contempo- 
taneous volcanic activity during the Richmond epoch. 
II. The Hornblende-diorites, Porphyries, and Granitoid Rocks. — 
These rocks are found in situ only in the third of tho island lying east 
ofa north and south line between Spanishtown and Port Maria in the 
Parishes of St. Andrew, St. Mary, Portland, and St. Thomas, and barely 
extending into the northeast corner of St. Catherine. 
They occur as dikes cutting through the Blue Mountain Series into 
the Oceanic Series, and as deep-seated interior masses from which such 
”a » 66 
dikes extend upward. These are the “granites,” “syenites,” *' diorites," 
and * porphyries," in part, of the Jamaican Reports, which contain many 
excellent detailed descriptions of their occurrence, but nowhere satis- 
factorily discuss or fully describe them as a whole. 
: The terms granite, syenite, and porphyry are used interchangeably 
m the Jamaican Reports for what is practically the same rock or dif- 
ferentiation of the samo magma, which being true, most of the rocks 
"Under discussion can be reduced to two classes, the first of which is the 
Above mentioned gneiss of porphyritio granitoid rocks, the second, horn- 
blende-diorites. According to Cross, the latter are “all simple normal 
granular rocks, with much more plagioclase than orthoclase, common 
Sreen hornblende, and some altered biotite. The quantity of orthoclase 
and quartz varies in different specimens.” In occurrence these rocks 
Cannot well be separated from one another, or at least have not yet been 
Separated, In St. Thomas-in-the-Vale? the syenitic, dioritic, and feld- 
Shathio porphyries seem to succeed one another to the westward. 
hile certain rocks are called granites throughout the Jamaican 
9ports? Sawkins in speaking of these has clearly noted that they 
differ from the true granites by the absence, abundance, or replacement 
of one of the constituent minerals. “It must, therefore, be understood 
a They have also been described by De la Beche, but in both cases without 
microscopic aid. 
: Jamaican Reports, p. 139. 
Ibid., p. 106. 
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