the Lower Pecos Valley, New Mexico. 425 



made along this dike but no appreciable mineralization 

 appears to have taken place other than a limited amonnt 

 of pyritization. 



This intrusive is a fine-grained, slightly porphyrinic, 

 bluish gray rock composed of augite and magnetite in a 

 matrix of lath-shaped feldspar crystals, showing typical 

 diabasic texture (fig. 5). Extensive secondary alteration 

 has taken place, the feldspars being highly kaolinized and 

 the augite almost wholly altered to secondary amphiboles, 

 consequently the rock is easily weathered giving rise to no 

 topographic feature other than a slight depression where 

 the dike cuts a resistant formation. The composition is 

 intermediate between the andesites and the basalts, but 



Fig. 6. — Dunlap sill. Looking east. 



neither megascopically or microscopically does it resem- 

 ble a normal basalt, so the intermediate term augite-ande- 

 site is adopted instead. 



Dunlap Sill. — About eight miles south of the post-office 

 of Dunlap there occurs a sill intruding the Red-beds, that 

 can be traced for almost four miles along its outcrop from 

 Sec. 1, T-4-S, R-22-E, through Sees. 32 and 33, T-3-S, 

 R-23-E, to Sec, 3. T-4-S, R-23-E, and throughout most of 

 the area it forms a prominent scarp facing north (fig. 6). 

 This intrusive like the dikes already described is not 

 directly associated with any center of igneous activity, 

 and it lies some fifty miles northeast of the extensive, 

 though decidedly more acidic, intrusive that forms the 



