394 Gordon and Graton — Formations in JVeiv Mexico. 



Silurian. — The general position of the rocks belonging in 

 this system, which have been identified only in the Silver City 

 region, and at Lake Valley, has already been stated. They 

 occur at the upper part of a series of limestones which 

 throughout the greater portion contain Ordovician fossils. 

 In this upper portion, near Silver City, close to the top, 

 are found pentameroids and other brachiopods which Mr. 

 Ulrich places in the Silurian and regards as the equivalent of 

 the Silurian horizon found in the Franklin Mountains near El 

 Paso. These rocks, as has been said, differ little if any in 

 appearance from Ordovician rocks underlying, and hence the 

 bottom limit of these Silurian rocks is not definite. Since 

 corals considered to be Ordovician have been found within a 

 hundred and fifty feet of the top of this limestone series, an 

 arbitrary thickness of 100 feet is assigned to the Silurian. It 

 is possible that small thicknesses of Silurian strata occur at 

 other places in the territory and have been overlooked ; but it 

 is more reasonable to suppose that they are absent, and the Sil- 

 ver City region thus stands as the only locality of Silurian 

 rocks in western ISTew Mexico or eastern Arizona. The silver 

 deposits of Chloride Flat, near Silver City, are situated in the 

 topmost portion of this limestone. 



Devonian. — Where Silurian rocks are absent, Devonian 

 strata rest directly upon the Ordovician limestones. In some 

 places there is a well-marked unconformity at this horizon but 

 in others no unconformity is apparent. Where the Devonian 

 rocks overlie the Silurian at Silver City, the succession of sedi- 

 mentation seems to have been perfect. The formation, which 

 has a maximum thickness of 165 feet, consists almost wholly of 

 shales and presents two well-marked divisions, the lower com- 

 posed of black carbonaceous fissile shales, and the upper of blue 

 shales which weather to a buff or brownish red color, and are more 

 or less calcareous. In Sierra County the black shales are from 

 100 to 200 feet thick, but at times vary greatly in thickness 

 within short distances due to the uneven character of the lime- 

 stone surface upon which they rest. JSTo fossils have been dis- 

 covered in these lower or black shales. At Lake Valley they 

 have heretofore been included in the Lower Carboniferous.* In 

 the same region the upper or blue shale division has a thick- 

 ness of 50 to 100 feet and in places is highly fossiliferous, not- 

 ably at Kingston and 2 miles east of Hillsboro, where the beds 

 contain brachiopods in profusion. Fossils were also known 

 to occur between the Santa Rita and Georgetown districts. 

 Dr. George H. Girty of the Geological Survey, to whom the 

 fossils were referred, states that the fauna is characteristically 

 Upper Devonian and adds that it is one of peculiar interest 



*Cf. Clark, E., loc. cit. 



