OF SANTO DOMINGO. 



121 



rock, apparently a conglomerate, but so far altered that the pebbles have lost their 

 individuality, and seem to be fused into the mass. Further to the south is a belt of 

 earthy, semi-talcose slates, rarely silicious, and lithologically identical with those 

 found at Guaraguano, in both cases overlying the limestones. In these slates, as at 

 that locality, quartz veins are abundant, and occasional copper stains are apt to 

 mislead the ignorant into the fallacious belief that the metal may be found. It was 

 probably this that caused the execution of some old mining work on this hill, the 

 history of which is now forgotten in the vicinity ; or the mine might have been 

 worked for gold-quartz. » The excavation is eighty feet long and about thirty feet 

 deep ; the width is from three to five feet, and a number of small pillars left standing 

 have served to keep the walls perfectly apart. 'No definite vein structure could be 

 detected either in the " country rock !' or even in the pillars. We descended to the 

 bottom of the work by means of the long cable-like roots of the Iliguey trees grow- 

 ing on the rocks, but failed to discover any trace of metal. 



The Magna drains the whole east side of the ridge of the Pico del Gallo, receiving 

 the waters of the arroyo del Gallo, the stream which runs down from the peak itself. 

 At the j)oint where this creek empties into the Magna we find the dividing-line between 

 the southern edge of the second belt, to be descril3ed further on, and the sedimen- 

 tary rocks which form the whole mass of the peak. In reality, the mountain owes 

 its prominence to the fact of its being the entire thickness of the Cretaceous system, 

 pushed up bodily by the syenitic rocks which reach the surface in broad belts both 

 north and south of it. Just at the mouth of the creek we found a light-colored 

 syenite, with small slender crystals of hornblende and large white crystals of feldspar, 

 and almost in direct contact, south of this, we encountered the Cretaceous beds 

 changed to a soft brown mica slate, and a little further on, where the action had 

 liot been so marked, they were a greenish slate, which, according to my notes, was 

 " seemingly micaceous." 



Still further south, along the Magna, on the third and last Ijelt encountered, and 



which crosses the spur of Pico Gallo between the peak and the main ridge, there is 



a great variety of rocks all occurring in the same mass, often having a considerable 



longitudinal extension ; that is to say, in an east and west direction, but very limited 



in a direction across that of the dyke. A marked illustration of this occurs in 



the rock with greenish feldspar at the crossing of the Magna. Although the Avhole 



syenitic mass of this belt appears to have originated in one intrusion this particular 



strip can l)e traced from an uidcnown distance east of the ri\-cr to beyond the Jicome. 



So, in the present case, a specimen of a white laminated, gneissoid rock, containing a 



little black mica arranged in layers, brought me by Mr. Speare li'oni southeast of 

 A. r. s. — VOL, XV. 2e. 



