GEOLOGY AND GEOLOGIC HISTORY. 67 



basal complex and is apparently conformable with the overlying Cercado 

 formation. It is probably of nearly the same age as the Baitoa formation. 



The Bulla conglomerate is exposed from the entrance to the gorge of Rio 

 Mao at Bulla downstream nearly to Hato Viejo, but its lateral extent has 

 not been traced. 



The gorge of Rio Mao at Bulla is cut in greatly folded and highly meta- 

 morphosed schistose shale (see PI. VII, C), upon which rest coarse cobbles of 

 the Bulla conglomerate. On the left bank of Rio Mao, at the lower ford at 

 Bulla (see PL IX), the Bulla conglomerate forms a bluff that extends below 

 water level. The conglomerate contains greatly decayed boulders of 

 many kinds of igneous and metamorphic rocks, some of them more than 

 30 centimeters long, which are not definitely sorted. 



The first bare bluff on the east side of Rio Mao below Bulla on the trail 

 to Hato Viejo is about 50 meters high and is separated from the river by a 

 broad fiat. The bluff is composed of beds of coarse conglomerate ranging 

 in thickness from 30 centimeters to 3 meters or more, interbedded with 

 layers of sand from 30 centimeters to 2.5 meters thick. The conglomerate 

 contains poorly sorted pebbles and subangular boulders of diorite, horn- 

 stone, schist, and other rocks of the basal complex, ranging from about 1 

 centimeter to 30 centimeters in diameter. The upper, more sandy part, 

 which appears to mark the transition to the Cercado formation, contains a 

 few poorly preserved mollusks (station 8529; for list see pp. 116-124). The 

 inter-fingering of sandstone and conglomerate is shown in Plate X. The 

 strata in this bluff dip northward at an angle of 8°. 



The thickness of the Bulla conglomerate is about 120 meters. 



CERCADO FORMATIOV. 



The name Cercado formation has been applied by Maury 1 to the fine 

 blue or gray silty sand exposed in a long double bluff (Bluff 3 of Maury; see 

 PL VIII and PL XII, A) on the west side of Rio Mao 5 to 7 kilometers south 

 of the village of Cercado de Mao, just above Paso del Perro and opposite 

 Hato Viejo. The strata which are exposed in bluffs at and near the village 

 of Cercado and for which the name Cercado would be more appropriate are 

 younger than the Cercado formation and are included in the Gurabo 

 formation. 



The height of the bluff at the type locality ranges from about 18 meters at 

 its lower end to perhaps 75 meters. It is cut through about 1 kilometer 

 above the ford by an arroyo. The cliff is nearly vertical, but at its base 

 there are several massive rounded ledges. The lower 6 meters of the bluff 

 consist of bluish sandy shell-marl that includes some layers composed prin- 

 cipally of Foraminifera of the genus Amphistegina. These little shells 

 are so abundant that the beds containing them resemble coarse sand. 



1 Maury, C. J., Science, n. 8er., vol. 50, p. 591, 1919. 



