GEOLOGY OF THE LOWER AMAZON REGION 731 



this Devonic is traversed by dikes of diabase. Here the sequence is 

 the following (zones (i ?) 2 and 3 are referred by the present writer 

 to the Erere formation, and 4 is the top of the Maecuru) : 

 Upper Carbonic. 



1. Black shales, in part sandy and micaceous. Locally fossilifer- 

 ous. Thickness estimated at from 15"^ to 20"^, but increases to the 

 north. Toward the top these beds are interbedded with, and finally 

 covered by, masses of " schalstein," attaining a thickness up to 40™. 



2. Thin-bedded to shaly sandstone, with hematite particles colored 

 rose to brown-red. Single thin zones are very rich in fossils. (See 

 f aunal list. ) Some interspersed beds are gray ; others, white. Thick- 

 ness, probably not less than 25"^. 



3. Alternating dark-gray to black sandstones and black bitumi- 

 nous and coaly shales. Toward the top occur rarely Orhiculoidea 

 lodensis, var. , and small Lingula. Thickness, about 20™. 



4. Black, tough, thin-bedded hornstones, with interbedded seams 

 of sandy or clayey beds. These weather to light gray, also red and 

 banded. Traces of fossils. Thickness, about 10"^. This zone is 

 correlated by Katzer with zone 4 of the Rio Maecuru section. 



Balance concealed. 



The above sections indicate that the Devonic of north Brazil is not 

 less than 100™ thick (about 325 feet), 75™ of which occur above the 

 base of the hornstone zone and 25"^ beneath the same formation. 

 This hornstone bed is provisionally suggested as the one distinguish- 

 ing between the Lower Devonic and the Middle Devonic. This is 

 done, because, on the one hand, it is the only place where a sharp 

 lithic difference exists in the section; and, on the other, on account 

 of the two distinct faunas occurring one beneath and the other above 

 the hornstone zone. Whether a time break exists here cannot be 

 determined from the published record, but the faunas indicate that the 

 one from the Rio Maecuru and Rio Curua, found beneath the horn- 

 stone, is to be correlated with the Oriskany, while that above the 

 same zone, or the fauna of Erere, is indicative of lower Middle Devonic. 



Before proceeding to a general discussion regarding the inter- 

 relation of the Devonic faunas of South and North America it will 

 be best first to note the peculiarities of the Amazon faunas. A 

 complete list of these faunas is given at the end of this paper. 



