86 A GEOLOGICAL SUKYET OF LANDS 



marls and sandstones of small grains of glauconite. This is a 

 phosphatic mineral, which has been worked extensively for the 

 market in beds of the same age on the Atlantic coast and used 

 as a fertilizer. In the upper part of the Austin chalk formation, in 

 the Upson cla}', and in the San Miguel beds, this mineral is present 

 in small grains as an original ingredient, which is continually 

 yielded to the soil forming on the surface. 



IGNEOUS ROCKS. 



During the tertiary' age, while the Upper Cretaceous strata 

 extended uninterrupted far to the north and before the land had 

 been elevated to its present level, some volcanic disturbances re- 

 sulted in the injection of what must have been extensive sheets of 

 basaltic rock, chiefly- among the strata of the Eagle Ford beds. At 

 least two or three thousand feet of sediments have since that time 

 been removed by erosion from the country north of Eagle Pass, 

 and the land has been cut down to a level mostly below the in- 

 truded rock, in the country north of the main line of the Southern 

 Pacific railroad. It seems probable that there was only one sheet 

 originally injected. This may have extended as one continuous 

 body from east of Turkej* mountain to west of Pinto mountain. 

 But the greater part of this sheet has been cut away with the 

 formations in which it lay, and at the present time we find only a 

 few small remnants of the original iatrusive. These remnants 

 are left on account of their effective resistance to weathering and 

 erosion, and we find them capping the highest pointsintheregion. 

 They are the dark rocks found on seven peaks which rise as pro- 

 minent landmarks on the uplands arouzid Fort Clark: Turkey 

 mountain, Elm mountain, Las Moras mountain, Pinto mountain. 

 Little Pinto mountain, Palmer hill, and another low hill two 

 miles north of Las Moras mountain. 



The rock is alike in all of these places. It is a dark basalt, 

 composed chiefly of plagioclase feldspar, olivine, augite, and 

 magnetite. It usually has a somewhat porphyritic structure, 

 but varies considerably as to coarsness of texture. In some 

 cases the crystals are readily visible to the unaided eye, while at 

 other points the rock appears compact and the crystals are 

 microscopic. Ever^'where the rock is fresh. It has undergone 

 practically no change from its original condition. This makes 



