GEOLOGY AND GEOLOGIC HISTORY. 73 



A cut near Canela, 15 kilometers from Santiago, on a road from Santiago 

 to Sabaneta, exposes soft greenish-gray shale, sandstone, and a little 

 nodular limestone containing many fossils (station 8726) . 



The Gurabo formation is exposed also at many other places in the valley 

 of Rio Yaque del Norte, especially in the vicinity of Santiago. Lists of 

 fossils obtained from the formation are given on pages 130-151. 



MAO ADENTRO LIMESTONE. 



The Mao Adentro limestone is typically exposed in the bluff on the right 

 (east) bank of Rio Mao opposite the village of Mao Adentro. The follow- 

 ing section is given by T. W. Vaughan: 



Section on Rio Mao opposite Mao Adentro. 

 Miocene (Mao Adentro limestone) : Meters. 



2. Conglomeratic limestone interbedded with bluish shale; about 

 five limestone ledges and five shale beds within 8 meters. 

 The limestone is a mat of corals, among which branching 

 foims are predominant. Station 8532. Total exposed 

 thickness in cliff face about 21 



1. Laminated sand and silts, originally bluish, oxidized yellowish 

 or brown. The sand is medium to coarse with a few small 

 pebbles 1.5 to 3 mm. long. There are interbedded thin 

 laminated beds of moie argillaceous material, which is well 

 and rather evenly stratified in layers from 6 mm. to 15 

 cm. thick. Station 8533. A few fossil corals 3.8 



The beds dip 8° N. 60° W. and strike N. 30° E. 



The Mao Adentro limestone forms the hogback ridge at Mao Adentro 

 and caps the ridge south of Cercado de Mao. The top of the Samba Hills 

 is probably formed of this rock. A view of the ridge on the east side of Rio 

 Mao is shown in Plate XIII, B. 



An exposure on Arroyo las Lavas at the crossing of the highway 

 from Santiago to Monte Cristi, several kilometers southeast of Navarrete, 

 seems to represent the Mao Adentro limestone. The strike of the beds in 

 the arroyo agrees with the alignment of the mountain front of the Cordillera 

 Septentrional (about N. 70° W.), and the apparent dip is about 80° N., 

 toward the mountains, but the beds are really overturned. This deforma- 

 tion is doubtless the result of the fault that separates the Miocene deposits 

 of the Yaque Valley from the Eocene and Cretaceous rocks adjacent to 

 them in the front of the Cordillera Septentrional. The material in the ex- 

 posure on Arroyo las Lavas is a mass of corals, chiefly of branching forms, 

 in a matrix of soft yellowish argillaceous marl. Associated with the corals 

 are a few mollusks. The species collected at this locality (station 8663) 

 are listed on pages 152 153. 



The Mao Adentro limestone caps a hill about 3 kilometers northeast of 

 Santiago, at the locality known as Gurabo Hills (stations 8723 and 8724). 

 The harder parts of this limestone are quarried by the Department of 



