Lee — Lower Paleozoic Books of Central New Mexico. 181 



Section of Rocks exposed in the Caballos Mountain three miles 

 north of Shandon, New Mexico. 



No. Ft. 



(1) Limestone, blue, massive, fossiliferous, in thick plates 



separated by dark colored shales (age, lower Pennsyl- 

 vanian) 600 



(2) Limestone, cherty in places, white to brown (age not 



determined) __ 500 



(3) Limestone, cherty, massive, cliff-making. The following 



fossils occur near the top : Rafinesquina cf. kinc/i, 

 Plectambonites saxea, Plectambonites n. sp., Favosites 

 asper, Zygospira recitrvirostris (Richmond mutation), 

 Rhyyichotrema capax (age, late Ordovician) 600 



(4) Shale, dark green, containing Obohis ( Westonia) Ston- 



eanus Whitfield, Obohis sinoe W alcott ? (The fol- 

 lowing fossils were found in talus derived apparently 

 from this horizon : Plectorthis dvsmopleura Meek, 

 Obohis sinoe Walcott, Lingulella acutangula Roe- 

 mer f (age, upper Cambrian) ,_.. 90 



(5) Quartzite. 10 



(6) Granite ? 



Cambrian sediments occur also in Cerro Cuchillo, a hill con- 

 sisting of rocks faulted and tilted steeply to the east, that 

 stands a few miles west of the Rio Grande near the northern 

 end of the Caballos Mountains. The rocks were probably 

 originally continuous with those of the Shandon region, but 

 the two exposures are 20 miles apart and are separated by a 

 zone of profound faults. Here, as in the Caballos Mountains, 

 the Cambrian sediments are about 100 feet thick and consist 

 of a ba^al quartzite resting on granite and overlain by green- 

 ish shale in which were found fragments of trilobites and the 

 same speeies of brachiopods found near Shandon. The shale 

 is overlain by cherty limestone similar to the Ordovician of 

 the Caballos Mountains but no fossils were collected from it. 



The Ordovician rocks of the Caballos Mountains are appar- 

 ently conformable with the Cambrian, but as shown below, 

 there is probably a considerable time-break between them. 

 They consist of massive cherty limestone and form a conspicu- 

 ous cliff 600 feet high. The rocks of undetermined age 

 above are less cherty and not so massive, but in some places 

 can be distinguished from the lower chert only by careful 

 examination, the two forming a single cliff 1100 feet high. 

 The fossils from the lower chert indicate Richmond or late 

 Ordovician age, and in the absence of evidence, the upper 

 chert also may be late Ordovician or it may be Silurian. 



The chert is overlain in the Caballos Mountains by 600 feet 

 of limestone of Pennsylvanian age. No fossils of intervening 



