220 GEOLOGY OF TRANS-PECQS TEXAS. 



rocks exist which have not been observed in the Quitman Mountains, so far 

 as they have been examined. Porphyritic rocks are found, however, resem- 

 bling those of the Quitmans. The surface rock of the Sierra Blanca Moun- 

 tains consists of a quartzitic material in sharp, angular fragments, varying m 

 size from 6 inches to more than 6 feet, evidently a metamorphic or semi- 

 fused sandstone, with occasionally a thin layer of crystals of hornblende. 

 These quartzites, which are broken nearly at right angles to the plane of the 

 original stratification, are devoid of petrifactions or impressions of organic 

 matter. They evidently cover the slopes and summits to a considerable 

 depth, coinciding in their present inclination with the mountain slopes. The 

 isolation, character, and the peculiar shape of the Sierra Blanca Mountains 

 seem to point to laccolitic intrusions as their origin. 



The four mountain cones of this group are decidedly newer than the 

 Lower Cretaceous rocks surrounding them. The dip of the Lower Creta- 

 ceous hills in the vicinity of the Sierra Blanca Mountains is the same as that of 

 the quartzite forming the slopes of these mountains, and the. saddle of Lower 

 Cretaceous limestone, with excellently preserved specimens of fossils, lying 

 between two of the mountains, and stratified horizontally, seems to indicate 

 that the upheaval of all the mountains of this group took place simultaneously 

 and probably gradually after its deposition. 



From observations made up to this time no connection can be found be- 

 tween the intrusive material of the Sierra Blanca cones and the eruptive 

 rocks of the nearest or northwestern ridge of the Quitman Mountains, and it 

 will require careful study and comparison with the Quitman Mountains, and 

 the distant ranges and groups to the northward, to justify final conclusions. 



The foothills east and south are Lower Cretaceous, with numerous and 

 extensive porphyritic intrusions. To the west there are more recent (Qua- 

 ternary?) beds, sloping gently towards the river and intersected by numerous 

 ravines and dry watercourses. The Sierra Blanca Mountains, with their 

 fragment-covered surface, and the foothills west of them show fewer distinct 

 ore outcrops, and are lacking in the tempting indications of ore deposits and 

 veins found in the Quitmans and their foothills. I regard this plainly visible 

 presence of ore in the Quitman Mountains, and the absence of such distinct 

 indications in the Sierra Blanca group, as additional evidence that they be- 

 long to separate upheavals, independent of each other, and also of the lacco- 

 litic character of the Sierra Blanca group. 



The Quitman Mountains, which have a general northwest trend, comprise 

 two separate parallel ridges, the northern of which is much the shortest. 

 The northern ridge is a granitic upheaval with porphyritic intrusions, having 

 in one place a small saddle of Cretaceous material overlying the granites. 

 The northwestern part of the second ridge is of a similar composition to 



