DESCRIPTIVE DETAILS : HAGAN FIELD 637 



recognized over wide areas by their lithologic characters and by charac- 

 teristic fossils. The lowest one, containing the Gastropod zone, so called 

 by Herrick and Johnson (77, page 175), in the liio Puerco field, is 

 found in a dark-colored shale 90 feet thick, and here, as elsewhere in 

 central New Mexico, contains limestone concretions. However, these 

 are not so fossiliferoiis as are those of tlie Gastropod zone on the Kio 

 Puerco. A few imperfect shells, mostly of the genus Inoceramus, were 

 seen in it, but none were collected. 



The Tres Hermanos sandstone (77, page 176) is recognizable in this 

 field, but it is only 5 feet thick where the section was measured. How- 

 ever, it increases to 15 or 20 feet in some places near by. It was ob- 

 served along the outcrop from a point about 2 miles south of Hagan, 

 northward to Pina Vititos, a distance of about 8 miles, where it disap- 

 pears under surface debris. It contains a few poorly preserved gastro- 

 pods and fragments of other shells. 



The main body of the Mancos shale overlies the Tres Hermanos sand- 

 stone. It was measured and examined with considerable care near 

 Hagan. The lower half of it consists of dark-colored shale containing 

 thin layers and concretions of limestone. The upper half is more or less 

 sandy and weathers to a brownish yellow color. These two subdivisions 

 were readily recognizable throughout the Hagan field. A thin limestone 

 about 175 feet above the base of the shale is full of shells of the species 

 Inoceramus labiatus, a characteristic fossil of the Greenhorn limestone. 

 A zone characterized by numerous limestone concretions occurs about 400 

 feet above this limestone and has been called the Cephalopod zone by 

 Herrick (77, page 177). These concretions are very fossiliferous and 

 yielded the fossils named in the accompanying section (lot number 

 7177). 



In the lower part of this concretion zone a petrified log 10 inches in 

 diameter was found. The interior portions of it are silicified and the 

 silica center is surrounded by an envelope of coal about half an inch 

 thick. Seventy-five feet above the top of the concretion zone the shale 

 contains thin layers of dark-colored limestone containing the fossils 

 named in the accompanying section as lot number 7178. These shells, 

 together with those from the concretion zone below them, belong to the 

 Benton fauna. Several of the species are the same as those that Johnson 

 (81) included in his Benton fauna of the Cerrillos field. The writer 

 visited Johnson's locality near the smelter at Cerrillos and is convinced 

 that the horizon represented there is about the same as that indicated in 

 the accompanying section 75 feet above the Cephalopod zone. 



At still higher horizons the Mancos shale contains large Inocerami 18 



