210 GEOLOGICAL RECONNAISSANCE OF THE DOMINICAN REPUBLIC. 



apparently without fossils, and shale that weathers to a faint reddish' 

 brown color. These beds slope valleyward at a low angle. They dip 

 southward in the northern part of the valley at an angle of 10° to 15°, 

 and dip northward at the few places where seen in the southern part. 

 The gravel consists of pebbles of limestone, diorite, and volcanic rock in 

 variable proportions, their character depending on the rocks in the moun- 

 tains that border the basin. 



The age of the gravel is not known, as no fossils were found within beds 

 recognized with certainty as a part of the formation, but data obtained 

 along Rio Yaque and farther south indicate that the formation is composed 

 of terrestrial or near-shore deposits of early Pliocene age, postdating the 

 late Miocene marine Cerros de Sal formation, which contains the beds of 

 gypsum and rock salt found along the south side of Enriquillo Basin. 



The gravel of the Las Matas formation of San Juan Valley, as has already 

 been stated, apparently rests unconformably upon folded and faulted 

 fossiliferous strata of Miocene age, which consist of dark-gray to bluish 

 friable sandstone and conglomerate, shale, and a little limestone. As the 

 Miocene rocks are largely concealed by the gravels in the valley they are 

 exposed only in isolated hills that rise above the valley floor, for the Mio- 

 cene rocks are slightly more resistant than the gravels. Sierra del Agua, 

 a low range of hills that forms the eastern limit of the basin, is made up 

 largely of these strata, typical exposures of which may be seen along the 

 San Juan-Azua road near Rio Yaque. 



The only other rock in San Juan Basin that is noteworthy in this con- 

 nection is basaltic lava of recent age, which lies in nearly flat sheets, capping 

 mesas and upland areas of the gravels. No attempt was made to find 

 the vents from which the lava issued. Jones, in his report on Haiti, 1 

 mentions having found in the Cul de Sac district, adjacent to Azua Prov- 

 ince, a well-defined crater from which extend basalt flows that occupy 

 depressions in the present surface and are evidently very recent. 



Local Details. 



San Juan Valley was entered from the northwest by the trail leading 

 from Banica to Las Matas. Short trips were made southward from Las 

 Matas and in several directions from San Juan. The mountain slopes on 

 the south side of the valley were not visited. As viewed from Las Matas 

 the lower slopes are seen to be white limestone resembling the limestone 

 on the north side of the basin, which is probably of early Tertiary age. 

 The higher mountain slopes are covered with timber that conceals the 

 rocks. 



In the vicinity of Las Matas there are low hills of the Las Matas forma- 

 tion, made up of clay, shale, and gravel with interbedded marly limestone. 



■Jones, William F., A geological reconnaissance in Haiti: Jour. Geology, vol. 26, pp. 728-752, 1918. 



