362 Keyes — Unconformity in Central JVeiv Mexico. 



A still more instructive section is displayed in the canyons 

 cutting the Chupadera Mesa, 50 miles east of Socorro. The 

 surface rock of the great mesa is horizontally disposed Cre- 

 taceous sandstone, rather massively bedded and yellowish in 

 color. The dissecting canyons are 300 to 400 feet deep. They 

 disclose the blue Carboniferous limestones in many places. A 

 number of enormous dikes of gray trachyte traverse the dis- 

 trict where the limestones are displayed. Along these dikes 

 are large deposits of iron ore. On either side of the dike the 

 limestone is abruptly upturned, often nearly to a vertical atti- 

 tude. The cross-section is shown in the subjoined cut (figure 2). 







"x 



^>N. 



h~~ ■ 1 







V 



A 



'V**"^s. 



' ZOO feet ' 







- <TV. 



^ v_ 







■■ v - — 



-l*2rciLL(JZOUS 



~-\ 



5 



S^pcttrhoniferous 









Fig. 2. Unconformity of Cretaceous on Carboniferous. 



At this particular point the Cretaceous beds repose horizon- 

 tally, while in a distance of a thousand feet the Carboniferous 

 limestones change in dip from horizontal to vertical. The 

 dike is 200 feet thick. The iron ore on either side is of vary- 

 ing thickness, and tills the jagged edges of the broken lime- 

 stone. 



With the recognition of an enormous erosion interval in 

 central New Mexico prior to the deposition of the Cretaceous 

 sandstones, the working out of the geology of the region is 

 greatly simplified. Many hitherto inexplicable phenomena 

 now find easy solution. The Cretaceous of the region is ail 

 Upper Cretaceous. Nowhere have the Lower Cretaceous beds 

 been found. 



The duration of the erosion interval was thus probably 

 equivalent to the sedimentation period of the entire Lower 

 Cretaceous of the Texas area. 



New Mexico School of Mines, Socorro, August 30, 1904. 



