EARLIER TERTIARY (EOCENE AND OLIGOCENE). 711 



proximity of the areas which they occupy and the evidence that the recent geologic conditions 

 have been sinailar in both. (3) Both formations bear about the same relation to a group of 

 igneous rocks which was in part contemporary with them and in part subsequently invaded 

 their beds. The differences in composition of these intrusive rocks are not greater than differ- 

 ences in igneous rocks Avithin the same area which are known to be nearly or quite contempora- 

 neous. (4) Finally, as pointed out above, it is possible and even probable that the two forma- 

 tions are nearly or quite continuous through the southern part of the upper San Juan VaUey. 



C 17. PANA]VEA. 



Howe *^^ states, in a report on the geology of the Canal Zone : 



The central portion of the isthmus along the canal line consists of rocks of igneous origin. 

 They are the oldest rocks of the region and have been observed at the surface in the central 

 area, between Mamei and Empire and northeast of the city of Panama. Diamond-drill borings 

 have indicated their presence also at other points, and it is beUeved that they underhe at greater 

 or less depth the whole territory included in the Canal Zone. 



To the northwest younger rocks of sedimentary origin occur, resting on the older igneous 

 mass and gently inchned toward the Caribbean Sea, so that ia passing from the interior toward 

 Colon successively younger beds are encountered. Their well-defined stratification and the 

 abundant fossil remains of marine organisms indicate that these rocks were formed by the 

 deposition of gravel, sand, and mud on the sea bottom, or in tidal estuaries where the mate- 

 rials gradually hardened into rock. Nearly the same conditions prevail on the Pacific side, 

 where, in addition to the older igneous and sedimentary rocks, younger igneous rocks occur. 



Dikes and large crosscutting masses of a basaltic nature have invaded all of the older 

 sedimentary rocks; they occur in greatest abundance in the central and southern parts of the 

 Zone and are believed to represent the last phase of active volcanism in the neighborhood. 



From its very characteristic occurrence in the vicinity of Obispo, tliis name has been given 

 to the formation about to be described that occupies a large part of the central region and 

 consists of andesitic breccias and associated lava flows. * * * 



The Obispo breccias, wliich are beheved to constitute a large part of the formation, are 

 of variable texture, usually being composed of angular fragments an inch or more in diameter. 

 No evidence was found that they were laid down or sorted by the action of running water. 

 The fragments consist of andesite of a variety of textures and show sUght differences in com- 

 position. The majority of the fragments appear to consist of pyroxene andesite, although a 

 fair percentage of hornblende andesites are present. The color of the breccia on fresh exposures 

 is a dark blue or green. The rock near the surface has been altered to a red clay. 



At a few places, notably in the vicinity of Empire, lava flows of massive andesite were 

 found associated with the breccias. The actual relation of the flows to the breccias is not 

 clearly shown, but it is believed that they were contemporaneous and that quiet outpourings 

 of lava alternated with the explosive eruptions that produced the breccias. The massive rock 

 is of essentially the same composition as that composing many of the fragments in the breccias, 

 but in most cases is too far decomposed to admit of exact determinations. * * * 



Judging from the extremely unconformable relations that exist between the Obispo forma- 

 tion and the younger rocks, it seems clear that following the eruption of the breccias the region 

 was thoroughly dissected by the erosive action of streams. The interval between the close 

 of the andesitic eruptions and the beginning of the next recorded epoch was of unknown extent, 

 and the original surfaces of the Obispo rocks were completely obUterated before the next suc- 

 ceeding formation — the Bohio — was deposited. * * * 



Probably the oldest sedimentary formation represented in the Zone is that which has 

 its most characteristic outcrops in the vicinity of Bohio. 



Near the raihoad station is a cliff, from which buUding stone has been quarried and which 

 rises 50 feet or so from the level of the flat on which the town is built. The rock here is of 

 a very even texture and at first glanpe strongly suggests a fine-grained volcanic breccia. It 



