STRATIGRAPHY 559 



faces/ although the fresh surfaces are dark gray. They are in thin 

 beds, are well laminated, and are characterized by numerous joints 

 approximately at right angles to the bedding. They are quite sandy 

 and' Were not observed to have any limy partings, inclusions, or concre- 

 tions, nor were any fossils found in them. They are so indurated as to 

 be more or less slaty in places. 



The fossils found near Arani show that this formation originated, at 

 least in part, under marine conditions. The abundance of the sand- 

 stone and the sandy character of the shales are evidence of near-shore 

 deposition, and possibly they are in part continental in origin. Absence 

 of conglomerates and coarse material argues against any pronounced 

 relief in the area rimming the basin in which the Arani beds were de- 

 posited, or at least supports the belief that the depositing currents were 

 comparatively slow-moving. It is thought that this formation accumu- 

 lated in the sinking basin of an epicontinental sea surrounded by feature- 

 less lowlands. Absence of limestones and scarcity of fossils suggest that 

 the climate was rather cold, although the muddiness or siltiness of the 

 water may have been responsible for scarcity of marine life. 



TOTORA FORMATION 



Overlying the Arani formation is a succession of sandstones and shales 

 that differ so much in appearance from the underlying beds that it is 

 unquestionably justifiable to consider them an independent unit, although 

 no sharp break was proven to exist, between them and the Arani beds. 

 For this succession of beds the name "Totora formation" is suggested, 

 as it is characteristically developed near the town of that name. 



The age of the formation is established as Devonian by a number of 

 fossil collections. These fossils are all marine and in places are ex- 

 tremely numerous. Brachiopods are particularly abundant. 



The total thickness of the Totora formation is estimated as 10,000 

 feet. This is an extremely rough approximation, as no detailed measure- 

 ment was possible when the observations were made. The rocks of the 

 formation were first seen about four miles east of Totora, and they 

 surfaced most of the area between Totora and Samaipata, 100 miles to 

 the east. 



The formation consists dominantly of dark shales, with thin beds of 

 clayey sandstones and quartzitic sandstones, and, near the top of the 

 formation, a few massive sandstones. Float from at least one limestone 

 bed was seen, although no outcrops were observed. 



The shales range in color from black through bluish gray to light 

 gray. A few beds with a reddish tint were seen, but they are very rare. 



