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GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



GRANULAR INTRUSIVE ROCKS. 



Diorite. — The granular intrusive rocks include none more siliceous than 

 quartz diorite or granodiorite. Most of the specimens are quartz diorites. 

 Rocks of this class are found in the provinces of Monte Cristi, Santiago, and 

 Pacificador. The commonest type is medium-grained, gray, granitic- 

 looking rock, rather rich in quartz, containing variable amounts of horn- 

 blende. Biotite is subordinate to hornblende in all the specimens and is 

 quite absent from some. Most specimens contain little or no potash 

 feldspar. Only one (D. C. 56b) contains enough to suggest its affinity to 

 granodiorite. In one specimen (R. 33) the usual interstitial position of 

 orthoclase is occupied by albite, which may be secondary after orthoclase. 

 The poverty of these rocks in potash, combined with their rather high 

 silicity, is noteworthy. 



One specimen has a gneissic texture induced by pressure (D. C. 72). 

 Two (D. C. 23d, D. C. 25) are tentatively grouped with the diorites; 

 they have suffered deep-seated alteration and are possibly epidiorites de- 

 rived from gabbro. The feldspar is abnormally sodic for rocks so rich in 

 dark minerals. 



Gabbro. — A single specimen of gabbro (D. C. 23c) was collected in the 

 Province of Santiago. It consists essentially of labradorite, augite, and 

 hornblende. The hornblende, which is more abundant than the augite, is 

 probably in part secondary. Some of the altered rocks just mentioned, 

 however, may have been derived from gabbro. 



Peridotite. — A specimen (D. C. 3) of altered peridotite, consisting mainly 

 of dull serpentine derived from olivine but sprinkled with satiny crystals 

 of bastite, was collected near the town of Puerto Plata, and a hornblende- 

 olivine rock was collected in the Province of La Vega (D. C. 23e). 

 Specimens of granular intrusive rocks. 



