hill: geology of Jamaica. 109 



large amount of silica. The rock is to he 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 the structure of the 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 Eichmond beds, 

 they are detritus derived from rocks which were originally made in 

 previous geologic epochs, and that there are no indications of contempo- 

 raneous volcanic activity during the Richmond epoch. 



II. The Hornhlende-diorites^ Porphyries^ and Granitoid Roehs. — 

 These rocks are found in situ only in the third of the island Ivinir east 

 of a 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 w^hich such 

 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 

 in the Jamaican Reports for what is practically the same rock or dif- 

 ferentiation of the same magma, which being true, most of tlie rocks 

 under discussion can be reduced to two classes, the first of which is the 

 above mentioned gneiss of porphyritic 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 

 green 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- 

 spathic porphyries seem to succeed one another to the westward. 



While certain rocks are called granites throughout the Jamaican 

 Reports,^ 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 



1 They have also been described by De la Beche, but in both cases without 

 microscopic aid. 



2 Jamaican Reports, p. 139. 



8 Ibid., p. 106. . , . . 



