lxviii GENERAL SUMMARY OF SCIENTIFIC AND 



region with those along the northern and eastern borders of 

 our great paleozoic basin. Near El Paso, the various mem- 

 bers of the New York system, which make up the Cambrian, 

 or so-called Lower Silurian series, are all met with, resting 

 on crystalline granitoid rocks, and dipping westward at a 

 gentle angle. At the base are 250 feet of sandstone, with 

 Scolithus, regarded as Potsdam, followed by about the same 

 thickness of gray limestone, holding forms like Archeceyathus, 

 and supposed to represent the Calciferous. To this succeed 

 450 feet of gray magnesian limestone, with much hornstone, 

 having apparently the fauna of the Chazy, and overlaid by 

 300 feet of limestones abounding in the characteristic forms 

 of the Trenton and Hudson River formations the Cin- 

 cinnati group. Upon these rests a conglomerate, irregularly 

 distributed, and sometimes 60 feet thick, made up from the 

 ruins of the two last-named formations, which may represent 

 the Oneida or Shawangunk conglomerate. It is instruct- 

 ive to find such a conglomerate, indicating a period of dis- 

 turbance, alike in Eastern New York, in Ohio, and on the Rio 

 Grande, coinciding with the conglomerate of the May-Hill 

 sandstone in England, and marking in all of these regions a 

 great paleontological break. Resting unconformably alike 

 on this conglomerate and on the underlying formations are 

 about 400 feet of gray crystalline limestone, with obscure 

 fossils, which may be Niagara, but are considered more prob- 

 ably of Carboniferous age. It will be remembered that far- 

 ther northward Shumard found Trenton and Hudson River 

 rocks overlaid by the Carboniferous. Limestones of this pe- 

 riod are widely spread over this region, and in some parts 

 rest directly on old crystalline rocks, while elsewhere the Car- 

 boniferous limestone is found, with a thickness of 600 feet, 

 resting upon 800 feet of sandstone, probably of the same pe- 

 riod. No evidence is found in this region of the great series 

 of strata containing the Medina, Niagara, Helderberg, and 

 Devonian faunas. 



Further information has also been obtained with regard to 

 the geology of the great desert known as the Llano Estaca- 

 do, the strata of which were supposed by Marcou to be Tri- 

 assic and Jurassic, but are now known to be cretaceous. As 

 seen at Castle Canon, there is at the base 50 feet of a red 

 sandstone without fossils, which is probably Triassic, fol- 



