GEOLOGY OF GUATEMALA AND SPANISH HONDURAS 519 
lized rocks such as would be expected in a pre-Cambrian terrane 
are absent. 
A block of almost horizontally bedded uppermost Oligocene 
limestone forms a flat-topped ridge 3 miles wide at Livingston. 
This block was probably faulted down against the Carboniferous 
limestones of the Puerto Barrios reservoir during the late Miocene 
folding of the region, but its present width may have been deter- 
mined by later movements. Through this block the Rio Dulce 
cut a narrow gorge during the latest uplift of the region; the gorge 
is 300 to 500 feet in width and 175 to 300 feet in depth. The fauna 
collected in the walls of the gorge consists principally of corals and 
oysters, but it is not a coral-reef formation .* 
Oligocene limestone, which may be called the Rio Dulce iime- 
stone, is undoubtedly more widespread in northern Guatemala than 
has hitherto been supposed. Limestones forming low hills on the 
north shore of Lake Izabal at Jocolo (Fig. 4) with a N. 50°-80° E. 
strike and apparently vertical dip resemble the Rio Dulce limestone 
lithologically and in obscure fossil content. Likewise blocks of 
limestone collected at Fuerte San Filipe, at the entrance to Lake 
Izabal, show spines and other fragments of echinoids on weathered 
surfaces and are probably part of the same formation. As Sapper 
mapped the Livingston-Sierra de Sta. Cruz region as Upper Cre- 
taceous,? part of his extensive Cretaceous area may be of Ter- 
tiary age. 
Slightly consolidated quartz gravels and clays containing casts 
of marine shells of Pliocene or Pleistocene age are found in the 
lowest of the terraces on which Livingston is built. These gravels 
are probably a part of the formation which is exposed along the 
Omoa and Tulian shore of Honduras, as already described. No 
evidence of folding was seen, however, in the Livingston exposures. 
Fossiliferous white clays containing chert pebbles and interbedded 
black lignite seams of Pleistocene or possible Pliocene age underlie 
the region between Sanhil (Sierra de las Minas), the Rio Dulce 
«Dr. T. W. Vaughan, of the U.S. Geological Survey, identified the fragmentary 
fossils as resembling those of the Emperador limestone of the Canal Zone (the Empire 
limestone of R. T. Hill). 
2 Peterm. Mitt., Erginzungsheft 27, Heft 127 (1899), geologic map. 
