152 



ON THE TOPOGEAPIIY AND GEOLOGY 



Mao, ill the upper part of the hill a1)ove I^Taranjo and Cana Fistula, l)ut elsewhere the 

 sandstone is abundant. I had an excellent opportunity of examining the deposit in a 

 section cut by a little stream running north, between the Guanajuna and the Mao. 

 On the road, beds of gray sandstone alteiiia.te with the above claystone, both, but 

 especially the latter, abounding in little Pectens. ]^orth of the road, following the 

 spur, the sandstone forms a cap to the entire hill, but has been denuded away to 

 different levels, presenting sometimes an appcai'ance of a series of two or three steps, 

 one above the other. Below this the gravel is exposed to the nearly vei'tical walls of 

 the ravine, cut through to a depth of about fifty feet. I found an a])pearance of 

 imconformability here that I have never been able to detect elsewhere. A little 

 exposure of bluish shale in the bottom in one place seemed to dip slightly while the 

 gravel was horizontal. The outcrop of sliale, however, was too small to be depended 

 on, and the apparent dip eastward is so unusual as to throw a grave stispicion on its 

 reliability. The gravel shows, by its included pebbles, whence it came. All of the 

 I'ocks of the Sierra — syenites, slate, jaspei', cptartz — are represented, and as might be 

 anticipated a little gold is also found. 'No fossils occur in it, but the sandstone with 

 which it is capped contains numerous specimens of some of the most characteristic 

 species. On the hill I found Coiius, Pleurotoma, Pecten, Oliva, and other genera 

 identical with species from the blue shale. . ' . 



On reaching the Mao River a series of four tei'races is seen. These are best noted 

 from the top of the ridge above IsTaranjo ; those on the west side, above Hato Yiejo, 

 being better preserved and more marked than those on the east side. Their aggregate 

 height is between two hundred and three hundred feet, and they are composed 

 entirely of this gravel, which rests northward' on the shale in the Samba Hills and 

 south at the Angostura or narrows of the Mao, it meets directly upon the upturned 

 edges of the highly metamorphosed green jaspery slates and conglomerate of the 

 Cretaceous. 



The gravel here acqtiires a development seen nowhere else, its total thickness 

 being probably not less than three hundred feet, while at Cercado, a few miles further 

 down the river, it dwindles to less than twenty feet. The little creek called the 

 arro3^o Guaraguano, emptying into the Mao fi'om the soutliAvest at Hato Yiejo, has 

 cut a section in these beds more marked than that on the Mao. The beds are there 

 shown to be perfectly horizontal, and are made up from top to bottom of river debris, 

 brought down by the Mao from the intei'ior hills. South of this the gravels cap the 

 ridge which runs west of and parallel with the Mao, gradually thinning out, but 

 continuing horiz()iit;d, the lower beds disappearing first, abutting against the rising 



