the Lower Pecos Valley, New Mexico. 421 



occur the more extensive though probably earlier intru- 

 sives that form the backbone of the southernmost exten- 

 sion of the Rockies. 



Railroad Mountain Bike. — Railroad Mountain dike 

 extends as a prominent ridge (fig. 2) from Sec. 4, T-8-S, 

 R-26-E to Sec. 28, T-8-S, R-31-E, Chaves County, a dis- 

 tance of 30 miles in a general east-west direction. To the 

 east it becomes lost in the sands before passing under the 

 Plains and on the west it abruptly disappears beneath the 



Fig. 2. — Bailroad Mountain dike. Looking east. 



river deposits just west of the Pecos and does not come 

 to the surface on the western side of the river. As indi- 

 cated by its name the dike forms a ridge sometimes sixty 

 to eighty feet high and a hundred or more feet in width, 

 much resembling an abandoned and partially dissected 

 railroad embankment. The width of the dike is remark- 

 ably constant being approximately 74 feet at both extrem- 

 ities (Rg. 3), which would indicate that the exposed por- 

 tion represents only a fraction of the entire extent of the 

 intrusion. Where the dike first appears just east of the 

 Pecos the trend is N 89° E, striking directly for Capitan 



