92 



ON TTTE T()POC411ArnY ANT> GEOLOGY 



the eruptive rocks of the range have so strong a family likeness that the objection 

 falls to the ground. 



The resemblance that runs through all the intrusive rocks of Santo Domingo is 

 much more marked than would be anticipated over so large an area. Little or no 

 true granite exists, but in its place occurs syenite, varying in the size of the crystals 

 and in the proportion of admixture, but in almost every case a compound of recog- 

 nizable grains of quartz, feldspar and hornblende. Usually the material is so grouped 

 that one ingredient preponderates but little over the other, and the mass presents the 

 ordinary mottled-gray appearance. But occasionally one mineral iiicreases at the 

 expense of the others, or perhaps disappears entirely. On the Jaina there is a locality 

 where the rock contains large crystals of hornblende, the interspaces being filled up 

 with smaller ones of quartz and feldspar. In some of the specimens collected by 

 me the hornblende is almost the only mineral visible. Mica rarely occurs, but in the 

 same vicinity is another locality where a yellow rock is composed exclusively of 

 quartz and mica, and yet anothei- which yields a mixture of quartz and feldspar only. 

 These, however, are very trilling local exceptions and form but a small percentage of 

 the whole. For more detailed descriptions of the variations which these rocks 

 undergo, I must refer the reader to the descriptions of the local geology. ' 



Besides the granitoid rocks, there remains one other yet to be described. In the 

 mountains north of Bani there is an outflow of porphyry, which I have never suc- 

 ceeded in discovering, although I have at various times encountered, perhaps in all 

 half a dozen, pebbles and boulders in the river. The rock is a dark-brown or l^hick 

 paste, in which are embedded crystals of feldspar an inch across. Despite three 

 years of constant, careful search I have never found this or any other true volcanic 

 rock elsewhere on the Island. 



An instance of the uniformity of the geological phenomena in the Antilles occurs 

 in the fact that similar rocks ejected at about the same period are found also in 

 Jamaica. The geological report of that island describes "metamorphosed secondary 

 shale, sandstone and conglomerate" underlying undoubted cretaceous beds cut 

 through by "dykes of intrusive diorite, syenite and granite." 



