GEOLOGY OF THE PROVINCES OF BARAHONA AND AZUA. 211 



The topography suggests a general northward dip of about 15°. About 

 6 kilometers southwest of Las Matas, near several large springs of sulphur 

 water, at a locality known as El Puerto, Rio Macasia emerges from a gorge 

 through limestone which strikes N. 75° E. and dips 10° to 30° northward. 

 The rock is regarded as probably older than Miocene. 



Extending eastward from Banica is a mountainous limestone ridge known 

 as Sierra de los Altos. Where crossed at a point about 6 kilometers east 

 of Banica the south side of this ridge consists of andesitic agglomerate and 

 tuff resting on the limestone and possibly constituting the basal member of 

 the Las Matas formation, which extends from here southward. Exposures 

 in arroyos show loosely cemented, coarse conglomerate interbedded with 

 sand and hematite-red clay beds. The dip is generally about 15° south- 

 ward. About 5 kilometers south from Sierra de los Altos is a small west- 

 ward-flowing stream known as Rio Cafia or Tocino. Eastward along this 

 valley outcrops of the fossiliferous Yaque group were found underlying the 

 Las Matas formation. These strata consist of sandstone, limestone, and 

 conglomerate striking about east-west and dipping steeply southward. 

 In these strata certain species of characteristic Miocene fossils, belonging 

 to the genera Conus, Turritella, and Area, are plentiful. Probably the 

 presence of these deposits in the midst of the valley is due to comparatively 

 recent faulting. This view is supported by the fact that in the same vicin- 

 ity the beds of the Las Matas formation are steeply inclined. 



The road from Las Matas to San Juan passes over a nearly level gravelly 

 country with numerous dry, shallow arroyos and no rock outcrops for the 

 first 10 kilometers east of Las Matas. At a distance of about 12 kilometers, 

 at a locality known as Punta Cafia, the road ascends to a bench about 10 

 meters high, which is capped by a basalt flow resting on gravel of the Las 

 Matas formation. From this bench, which forms the divide between the 

 westward-flowing waters tributary to Rio Artibonito and the Rio Yaque 

 drainage, the road leads over low gravel hills and thence across the extensive 

 Sabana de San Tome* to the town of San Juan. 



For a long distance up Rio San Juan the country is a gravelly plain, across 

 which the river flows swiftly in a shallow, bouldery channel bordered by low 

 gravel embankments. A short distance above Carpintero massive buff 

 to white limestone appears in the river channel, and a little farther up- 

 stream it forms hills. The rock is more or less brecciated, is finely crystal- 

 line, and dips steeply northward. This limestone is in fault contact with 

 quartzitic sandstone and hard bluish shale that dip steeply northeastward. 

 The recent gravels along the river consist largely of huge boulders of 

 andesite but include some boulders of diorite and limestone. The mountain 

 slopes to the west in the same vicinity consist of limestone. No identi- 

 fiable fossils were found, but it is believed that all the hard rocks seen along 

 this part of Rio San Juan are of pre-Miocene age. 



