480 O. B. Richardson — Paleozoic Formations. 



whitish magnesian limestone approximately 1000 feet thick. 

 It overlies the Montoya limestone apparently. conformably, 

 although in one locality fragments of the underlying limestone 

 included in the Fusselman is evidence of an unconformity. 

 Throughout the greater part of the formation fossils are scarce, 

 but at a few horizons they are very abundant. The common- 

 est form is a species of radially plicated pentameroid shell 

 which, with Amplexus and Favosites, determined by Mr. 

 Ulrich, proves that the upper Niagaran stage of the Silurian 

 is here represented. Gordon and Graton* have recently found 

 Silurian fossils in the Silver City region and at Lake Valley, 

 New Mexico ; and Taff's Hunton formation in Oklahomaf 

 also contains a Silurian fanua. But with these exceptions the 

 Fusselman limestone is the only known occurrence of rocks of 

 Silurian age in southwestern United States.:}: 



C ARBONIFEROUS. 



Hueco Limestone. 



Neither the Devonian nor the Lower Carboniferous, so fat- 

 as known, is represented by sediments in trans-Pecos Texas, 

 and the Silurian, where present, is overlain by the Upper Car- 

 boniferous. The Hueco limestone outcrops in an area of sev- 

 eral hundred square miles in trans-Pecos Texas. It underlies 

 the Diablo Plateau, a large area between the El Paso and Van 

 Horn quadrangles, and outcrops in the Sierra Diablo, Finlay, 

 Hueco and Franklin Mountains. The Hueco is a rather homo- 

 geneous gray limestone, generally massive, though in places it 

 is thin -bedded. It is comparatively free from chert and differs 

 from the limestones of Silurian and Ordovician age in that it 

 contains little or no magnesia. Although the limestone is pre- 

 vailingly gray, there are local variations in color from light 

 gray to almost black. 



In the Franklin and Hueco Mountains the Hueco limestone 

 immediately overlies the Fusselman limestone apparently con- 

 formably in spite of the fact of the great hiatus indicated by 

 their ages. But in the Van Horn region a well developed 

 basal conglomerate averaging approximately one hundred feet 

 in thickness and composed of pebbles of all of the pre-Car- 

 boniferous formations is present at the base of the limestone, 

 which rests with marked irregularity on the underlying forma- 

 tion. The Hueco limestone generally is overlain by Pleisto- 

 cene debris, but in a few areas, notably in the Finlay 

 Mountains, and also eight miles northwest of Van Horn, it is 

 directly overlain by Cretaceous strata. The total thickness of 



* Quoted above. 



f Tishomingo Folio, U. S. Geological Survey, 1903. 



% Kindle, E. M., this Journal (4), xxv, pp. 125-129, 1908. 



