54 Shimer mid Blodgett — Mt. Taylor Region, New Mexico. 



ancient volcanoes. After leaving the Albuquerque mesa (Ter- 

 tiary) the only sedimentary rocks crossed were grayish brown 

 Cretaceous sandstones and shales. These strata have the slight 

 but prevailingly northerly dip of the plateau province, except 

 where given a westerly dip through the influence of the Naci- 

 miento mountain uplift to the east. This slight dip helps to 

 make the country one of flat-topped mesas and broad level val- 

 leys ; to this general appearance the old level flood plain of the 

 Rio Puerco contributes. The mesas are frequently capped with 

 lava flows. 



Projecting up through these strata in the Puerco valley, west 

 and northwest of Prieta mesa, are the many volcanic necks and 

 dikes for which, this region is famous.* 



These strata are mostly unfossiliferous, but when remains of 

 organisms do occur they are present in considerable numbers ; 

 examples of such fossil-bearing horizons are the "gastropod 

 zone" and the " cephalopod zone," as given in the generalized 

 sections of Herrick and Johnson. f 



In the region traversed the most conspicuous zone was a well- 

 marked bed of friable shale, frequently 25 to 50 feet thick, 

 carrying calcareous concretions (septaria) and bearing both in 

 the concretions and in the surrounding bed an abundant fauna, 

 consisting mainly of well preserved cephalopods, gastropods and 

 pelecypods. 



The concretions average \\ to 3 feet in diameter and are 

 seamed by dark calcite bands. They consist of a yellowish-brown 

 shale with a calcareous cement. 



In all the sections examined this zone occurs between beds 

 of a dark, friable, easily eroded shale. The shale above has a 

 thickness of 50 to 75 feet and is capped by a 5 to 10-foot bed 

 of a brownish yellow sandstone ; this latter, being more resistant 

 than the dark shale below, caps the cliffs. The septaria zone 

 shows at times as the top of a cliff but more usually as a line 

 of small hummocks, due to the fact that the septaria weather 

 more slowly than the embedding shales. 



Upon this series of beds rests to the west about 100 feet of 

 black shale, capped by about 50 feet of very resistant sandstone. 



These characters of the septaria zone remain approximately 

 constant wherever examined, — the northwest corner of the 

 Albuquerque sheet, the southwest portion of the Jemez sheet, 

 and the eastern part of the Mt. Taylor sheet. The more detailed 

 descriptions of the various portious are given below. 



* A paper on these necks by Douglas W. Johnson will appear in the bulletin 

 of the G. S. A. 



+ C. L. Herrick and J). "W. Johnson. The geology of the Albuquerque sheet, 

 Bull. Univ. New Mexico, vol. ii, part 1. 



