ON THE TOPOGRAPHY AND GEOLOGY 



the gradually thickening margin of the savana gravels made up entirely of debris of 

 the rocks of the Upper Jaina. vStill farther sonth in the savana of Santa Rosa these 

 gravels with their red sand, angular fragments of quartz and the little streaks of 

 black iron sand in every little rain-wash remind a Californian irresistibly of the foot- 

 hills of the Sierra ISTevada. They are in places slightly auriferous, and though not 

 probable, it is yet not impossible, that here if any where spots may be found where 

 " h} draidic mining " might be prosecuted with success. I say not probable, because 

 the small amount of gold in the present bed of the river seems to indicate that the 

 quantity would be proportionately still less when spread over so wide an {irea. And 

 yet the "black sand" although often found by itself, nevertheless the invariable 

 accompaniment of placer gold, is by no means scarce. 



South of the savana of Santa Rosa the red sandy matrix of the gravel becomes 

 gradually calcareous and though pebbles are yet occasionally found they are more 

 and more isolated until they also finally disappear, and the coast limestone is reached 

 just below the savana of La Venta or south of Managuallaba Creek. 



West of the Jaina, below the mouth of the Mano, is a hilly region of slates, 

 bounded on the west by the syenites which extend across to the Upper isigua. These 

 hills ai"e traversed by numerous dykes and are full of quartz veins. The slates are 

 an inextricable mixture of green, gray, brown and black claystones variously colored 

 jaspery slates and some more or less talcose. Among the first I found a couple of 

 miles below the mouth of the Madrigal, a black I'ock slightly arenaceous, very com- 

 pact and full of white grains. This is certainly a metamorphic shale, and yet, from 

 its appearance in hai'd specimens, might be mistaken for a porphyritic rock. A 

 similar material was found not rare on the Nigua River. The quartz veins are usually 

 very small, hardly ever over a few inches in width and seem to be limited also in 

 horizontal and vertical extent. A few however can be measured by feet rather than 

 inches, and Mr. Spear spent some weeks in making an experimental opening on 

 one of nearly three feet wide to ascertain its character. The result of his exca- 

 vation was to disclose a vertical vein with well-defined margins intercalated in the 

 shale. The quartz yielded a little gold by the ordinary miner's test of grinding 

 in a mortar and washing the powder. It is doubtful, however, whether the quantity 

 would have been sutficient to warrant earnest minint>'. 



On the ridge ovei'looking the mouth of the Mano at the head of two streams, 

 called the Anones and Caballo, which unite and run into the Jaina a mile or more 

 below the mouth of the Madrigal, is the largest quartz vein I have seen on the Island. 

 It cro})s out on the top of the ridge and its down-hill side is so covered Vv^ith soil and 



