E. Howe — Geology of the Isthmus of Panama. 235 



second area, in the vicinity of San Pablo and Tabernilla, is 

 probably more restricted than that of the Pacific side. The 

 rocks are best exposed along the Chagres River near San 

 Pablo, bat the hills extending northward from Barbacoas, 

 where the Panama Railroad crosses the Chagres, are composed 

 entirely of these acid pyroclastics. Their exact northwestern 

 limit is unknown but probably lies at some point between 

 Buena Yista and Tabernilla. Similar rocks are found in the 

 vicinity of Gatun interbedded with the sediments at that 

 locality. 



Sedimentary rocks. — A little more is known in regard to 

 the distribution of the sedimentary formations. So far as my 

 observations go, the Culebra beds are restricted to the occur- 

 rences in the Culebra district with a probable extension as far 

 south as La Boca suggested by borings. On the northern side 

 the sediments from the Bohio formation up to the Monkey 

 Hill have a much wider distribution. The sediments extend 

 from Limon Bay along the line of the canal to Bohio at least, 

 and remnants of the Bohio formation have been found as far 

 south as Matachin. The subdued topography characteristic 

 of areas of these young sedimentary rocks extends in a north- 

 easterly direction from Colon almost to Porto Bello and 

 throughout this distance I believe that sedimentary rocks are 

 the only ones represented at the surface. Between this coast 

 belt and the interior, where the limestones of the upper 

 Chagres are exposed, is a region of some elevation concerning 

 which there is no information whatever except quite near the 

 canal line, at Tabernilla and Frijoles, where poorly exposed 

 sediments containing lignite are known to occur. Southwest 

 of the canal line and along the Atlantic slope sediments have 

 been traced for nearly twenty miles up the valley of the 

 Trinidad River. The rocks of this region are of the same 

 character as the Bohio or Monkey Hill formations; they are 

 exposed at a few places along the water's edge, but the rock 

 at these outcrops is in an extremely decomposed condition and 

 no fossils were discovered. On the Pacific coast southwest of 

 the city of Panama and beyond the range of hills composed 

 of basic intrusives is the broad low valley of the Chorrera, 

 that, on account of its very subdued topography, looks as if it 

 had been eroded in sedimentary rocks. The Chorrera River 

 rises near the head waters of the Trinidad and it is not improb- 

 able that the sedimentary rocks extend across the divide and 

 down the Chorrera to the Pacific. According to Lull," coal, 

 probabh 7 a lignite, occurs in the drainage of the Rio Indio, 



* Eeports of Explorations and Surveys for the location of Interoceanic Ship 

 Canals through the Isthmus of Panama, E. P. Lull, U. S. N., Washington, 

 1879. Pp. 30-32. 



