502 O. H. HERSHEY GEOLOGICAL RECONNAISSANCE IN NICARAGUA 



rocks seen farther downstream, so that the latter may be andesite instead 

 of basalt. 



Andesite is present in the banks for several miles upstream, but the 

 rock which produces the Lalacapisa rapids, where examined in the chan- 

 nel, is a black conglomerate with many quartz pebbles. Above the rapids 

 for about a mile the banks show only the gravel and brown sand of the 

 Modern alluvium. 



Then the stream exposes on the north side a fine section of the varie- 

 gated silts and fine quartz gravel of the older alluvium. From the top 

 of the 30-foot bank a pine-clad, much-logged plain extends back from 

 the river. At the next good exposure of the Modern alluvium it seems 

 to rest on a much harder formation of dark gray conglomerate. I sur- 

 mise that the dark-colored conglomerates seen at many places along the 

 river, as at the Lalacapisa rapids, are merely portions of the Modern 

 gravel cemented by limonite. 



Dark rocks exposed on the south side of the channel at Wirapani are 

 probably andesite with flow structure. More of it is exposed upstream, 

 but it nowhere rises higher than the floodplain, so far as can be seen from 

 the river. Black rocks exposed higher up resemble basalt, even to an 

 imperfect columnar structure, but we did not land on them. It is prob- 

 ably safe to say that this country has a formation of lava sheets (and 

 probably some tuff beds) prevailingly andesitic, though more basic and 

 more acid rocks may be present in the series. 



At a bend, probably about 10 miles below the mouth of the Waspuc 

 Eiver, we got our first view of the mountains of the interior. There 

 seemed to be a peak and sharp ridge rising abruptly from the low plain, 

 but the view could not be depended on in this respect. 



In a bar of coarse gravel several miles farther upstream more than 50 

 per cent of the pebbles and cobbles were of white quartz. Many varieties 

 of andesite were the most abundant in the remainder. There were also 

 porphyries, diabase, diorite, red and brown chertlike rocks, probably a 

 little gabbro and basalt, some chalcedony, a hard quartz conglomerate, 

 an andesite tuff, a finely foliated gneiss, a black slate, jasper, and an 

 amygdaloid, but no granite, limestone, sandstone, nor fossils. 



Black rocks, presumably andesite, are scattered at intervals nearly to 

 Suhie, where the "Pine Eidge" country appears again. Here the bank 

 exposes white gravel (false-bedded near the river level) sufficiently in- 

 durated to stand in a vertical bluff and probably the same formation as 

 at Saclin. Then the river swings out into the Modern floodplain. The 

 deposit has changed very little in many miles. The gravel is moderately 

 coarse, of a light yellowish color, and usually rises one-third or one-half 



