150 



OIS^ THE TOPOGIJAPHY AXD CIEOLOGY 



tainiiig- much fossilized wood, sometimes showing tubes of teredos. Over the dark 

 shale there is a bed of the yellow shale similar to that near Gnayubin, and at the top 

 of the section a little white limestone, similar to that which caps so much of the 

 Monte Cristi range, and the equivalent of that of Cevico and elsewhere, A broad 

 depression in the strata, almost too shallow to be dignified by the name of a synclinal 

 axis, causes the older beds to nearly disappear ; but in the bluff at Santiago, cut 

 through by the Yaqui on one side and hj the little gully called the JS^ivaje which 

 empties into it, a good section is obtained again, here entirely in the blue shale. The 

 term "i^^ivaje shale" is ill chosen, the little stream after which it is named being a 

 mere gutter, while Santiago, immediately adjoining the stream, is a place of import- 

 ance and much more worthy of applying its name, should a distinctive title be con- 

 sidered necessary; or a dozen localities could have been selected whose names would 

 have been equally distinctive and much more suitable. 



At the mouth of the Bao (or Cibao) the dark shales are seen in contact Avith the 

 underlying coarse sandstones just mentioned. 



At Santiago, a vertical section of sixty feet through the blue shale shows its 

 bedding perfectly. It is j-arely horizontal in the valley, but the dips are invaria])ly 

 so low that they are hardly woi'th noting. Here it is a few degrees to the south. 



In the valley of the Yaqui, after leaving the immediate vicinity of the town 

 where the low hills run down to the river bank, there are very few exposures. At 

 some distance from the river occasional outcrops can be seen, some in the beds of 

 streams, others peeping through the soil ; but in the foot-hills of the mountains and 

 through the Samba range we have excellent opportunities of studying the formation. 

 A rare circumstance in this coinitry occurs in the existence of a few terraces south 

 of the Yaqui. Those on the Mao will be noticed hereafter, and bordering the river 

 on the " outer " Savaneta road, within a few miles of Santiago, the terraces are 

 extremely well marked with bluff fjices of river dehris, which I estimated at eighty 

 feet in height. The banks of the river are made up of pebbles and gravel derived 

 from the rocks of the sierra and from the sandstone strata of the Miocene ; but no 

 exposures occur of beds in situ. ' • 



South of this, far back in the hills at the crossing of the Bao, the conglomerate 

 beds of Angostura reappear, hei'e of a dark red color and with the pebbles usually 

 very small, cemented by a calcai'cons matrix. While these conglomerate beds and 

 the adjoining sandstone strata have a high northeast dip, the overlying beds further 

 north repeat the condition of aftaii's along the Yaqui, and fall gradually to nearly a 

 horizontal. 



