the Lower Pecos Valley, Neiv Mexico. 427 



place, some of which are of considerable extent. The gen- 

 eral composition of these intrusions, which are predom- 

 inantly sills, is that of an andesite, 7 and the sills are con- 

 formable with the stratification being also highly folded, 

 showing conclusively that the intrusion took place prior to 

 the folding. Andesite sills were also found in the red 

 shale series lying just under, and approximately conf orm- 

 ble with, the San Andreas limestone. 



Sills and Dikes in the Sierra Blanca Series. — The Sierra 

 Blanca coal-bearing series which is regarded as Upper 

 Cretaceous or possibly of post-Laramie age is widely 

 intruded by sills and dikes of the general composition of 

 those found near Capitan ( diorites and monzonites ) . Just 

 east of Capitan on the Lincoln road an andesite dike 

 trending N 24° E and about 15 feet thick crosses Salada 



Fig. 7. — Generalized cross-section from Capitan Mountain to Agua Azul, 

 showing the crumpled Yeso strata underlying the relatively undisturbed San 

 Andreas limestone. Scale 3 inches to the mile. 



Creek, cutting the massive sandstones of the lower part of 

 the series. Just west of Capitan on the highway to Cari- 

 zozo the coal-bearing upper series is intruded by dioritic 

 sills twenty -five or more feet in thickness. The series has 

 also been locally faulted and the coal in many places shows 

 the effects of the intrusion. 



Summarizing the above, the intrusives of the Pecos 

 Valley that are apparently of related origin and similar 

 age, inasmuch as they are generally similar in composi- 

 tion and mode of occurrence, are seen to occur not in any 

 one formation but in every series represented in the area, 

 with the exception of the recent sediments, from the Yeso 

 beds near Capitan to the Upper Permian Red-beds just 

 west of the Cap-rock, and the Upper Cretaceous or possi- 

 bly Eocene coal-bearing formations of the Sierra Blanca 

 series. 



^ 7 The term andesite is used by the writer to include also the basic ande- 

 sites (augite-andesites), classified as basalts by some writers. 



