EARLIER TERTIARY (EOCENE AND OLIGOCENE). 709 



Regarding the age of the formation, Sievers has no definite data, as he found 

 no fossils. The Caroni series of Wall was placed in the Miocene on fossil evidence, 

 and Sievers regards the Cerro de Oro terrane as probably equivalent to it. On the 

 other hand, he is not sure that it should be separated from the Cretaceous, and in 

 the absence of fossils its age remains undetermined. 



C 16. NICARAGUA. 



Hayes *^^ states that the Tertiary rocks along the line of the proposed Nicaragua 

 Canal are the oldest certainly known in that section and comprise two formations, 

 the Brito and Machuca. 



The [Brito] formation presents considerable variety in its lithological composition, but it 

 has not yet been sufficiently studied to permit of its subdivision, even if this may be eventually 

 possible. Much the larger mass of the formation consists of somewhat calcareous nonfissile 

 shale. When fresh this is bluish gray and weathers to a yellowish or brownish color. Dis- 

 tributed through the shale are numerous beds of sandstone. These are also somewhat calc;> 

 reous and doubtless contain a considerable proportion of volcanic ash. The sandstone beds 

 vary in thickness from a few inches to 2 or 3 feet and occur singly or in groups. * * * These 

 sandstones, like the shales, are blue when entirely fresh but are always weathered at the surface 

 to some shade of yellow or brown. * * * They occur most abundantly near the Pacific 

 coast and are well exposed in the headland northwest of the Rio Grande Valley at Brito. The 

 beds here have a general though somewhat variable dip to the southwest, hence the higher 

 portions of the formation miake the cliffs along the Pacific. • This seems also to be its most 

 variable portion. In addition to the shales, which constitute its greatest bulk to the eastward, 

 it here contains also beds of sandstone, conglomerate, and coarse breccia on the one hand, and 

 on the other marly beds and lenses of pure limestone. Forming a part of the headland south 

 of Brito is a bed of hmestone something over a hundred feet in thickness. Small outcrops of 

 this bed, or one very similar, have been noted at several other localities to the eastward in 

 the Divide Hills. Its limited extent is due in part to erosion, since the dip of the bed would 

 carry it above the tops of most of the hiUs to the eastward, but it is doubtful if its original 

 extension was very great. Several of the limestone outcrops noted are probably small lenses 

 in the shale and not connected with the more continuous bed at Brito. A portion of this lime- 

 stone has a pecuHar concretionary structure, some of the concretions attaining a diameter of an 

 inch and a half, while other portions of the bed are oolitic. Immediately west of this exposure 

 of limestone, forming a group of islets nearly covered by high tide, is a very coarse volcanic 

 conglomerate or breccia. The larger fragments are a foot or more in diameter and quite angular, 

 and from this extreme they grade downward to small pebbles, some of which are well rounded. 

 The present relations indicate that the conglomerate is the stratigraphical equivalent of the 

 limestone, replacing it within a few yards. In some places the two rocks are seen to merge, the 

 hmestone containing numerous angular fragments of volcanic rock. At other points along the 

 coast both north and south of Brito, similar conglomerates occur. Their bedding is extremely 

 irregular, and they afford evidence of having been formed rapidly and near the source from which 

 their constituents were derived. While it is possible that the source of this material may have 

 been to the eastward, it seems much more likely that it came from volcanic vents to the south- 

 west, from volcanoes which have been entirely removed by the .waves of the Pacific. * * * 



The exposures of the Brito formation are so infrequent and the dips are so variable that 

 no satisfactory measure of the thickness of the formation can be obtained. Taking the observed 

 dips between the Pacific coast and the lake shore the thickness exposed is estimated at about 

 10,000 feet. This of course is not the total thickness of the formation, since the bottom is not 

 exposed at the axis of the anticline. Also the formation has undoubtedly suffered an unknown 

 but considerable diminution in thickness by erosion, and there are no data for determining the 

 thickness of strata which have been removed from the highest beds now observed. * * * 



