188 , ON THE TOPOriRAPHY AND GEOLOGY 



fine grain and dark in color. At tlie mouth of the Jarjiei 1 observed sjx)ts where the 

 cjuartz was unusually clear and glassy ; in another place 1 found a little mica. Some- 

 times the syenite is flesli-colored or salmon-colored, owing to a more or less pinlvish 

 feldspar ; but usually it is some shade of gray or even nearly white. In the river I 

 found a gray rock resembling the peculiar porphyry-looking syenite ah-eady described 

 from Mount Basimo, on the. Jaina, but diflering in the smaller size of the enclosed 

 crystals of feldspar and quartz. The Majagual Creek, which empties into the Nigua 

 a mile below the Jamei, also runs through these rocks, there usuall}^ of a dark color, 

 though sometimes almost pure white. About a mile from its mouth there is a fall, 

 at the line of juncture between the slates and syenites, where excellent examples of 

 the contact can be obtained. Here, as on the Jaina, the two materials are completely 

 fused together so that they cannot be separated by the hammer. In ascending the 

 hill from this point the sedimentai'y rocks are found overlying the eruptive, limestone 

 capping the summit, with a low southern dip. Fragments of the latter are very 

 abundant both on the hill side and in the caiion, as well as parallel with this point in 

 the ISTigua, and although I obtained fossils abundantly here and in the deljvis in the 

 main river, I could never find the fossiliferous bed in place on the latter stream. For 

 a mile or more below the line of contact dykes cut through the jaspery slates, which 

 are here very much disturbed and seem to be more or less veilical. Approaching 

 Monte Mateo the limestones are found to dip northward at high angles, forming a 

 narroAV sj'nclinal. They are here all highly- altered and vary in color from dai'k gray 

 to pure white ; a sure criterion of the amount of alteration of this rock on the Nigua 

 is to be found in its color. When nearest its original condition and full of fossils it is 

 brick-red, and breaks with an earthy fracture. The fossils, it is true, are so far 

 changed that they are reduced to a highly crystallized spar, rendering it next to 

 impossible to extract them. ; but the rock shows no signs of change. But as the 

 metamorphism progresses, the material becomes lighter in color and more compact and 

 varies thi-ough dark flesh-color to all the shades of pink or gray, and the most 

 thoi"oughly altered specimens are perfectly white and even traversed by little seams, 

 like threads, of calc-spar. In no c^se, hbwe^^er, are they gi'anular, or do they approach 

 even remotely to the structure of marl)le. . . 



In this vicinity there are numerous signs of minerals. A little gold, hardly worth 

 mentioning, is found above Monte Mateo. On the Majagual, near the syenite a little 

 vein of magnetic iron ore, containing a small quantity of copper and some gold, was 

 opened a few years ago under the belief that the iron was gray copper ! Several 

 little copper veins ci'op oi]t in the bluffs of the river above the mouth of that ci-eek, 



