MEXICAN BOUNDARY LINE. 109 



The specimen marked " Isolated knobs near Frontera," consists of feldspar and olmne, and 

 is a modern igneous product. 



The specimen marked " Granite, north of Frontera," is a similar aggregate, more compact 

 and containing scales of mica. 



The mountain northeast of Frontera is partially composed of reddish feldspar with small grains 

 of crystalline quartz ; a kind of porphyritic rock. The position of this rock is remarkable and 

 highly interesting. A sectional sketch of the mountain by Dr. C. C. Parry, represents it as 

 resting on the upturned edges of the strata of carboniferous limestone, which form the base and 

 greater portion of the mountain, and dip at an angle of 45°. The granitic aggregate rests on 

 the sloping sides of the mountain, in the direction of the dip. At a little distance from this 

 point, and appai'ently resting on the latter rock, occur cretaceous strata, highly inclined, as if 

 the igneous mass had been forced out near the junction of the carboniferous and cretaceous beds. 

 In other instances the igneous beds rest on the cretaceous deposits ; leaving no doubt that the 

 eruption took place subsequent to the cretaceous period. 



The specimens from 35 to 41 inclusive, present the characters of the different members of a 

 section across the Tertiary and cretaceous strata, to the upper carboniferous limestone ; with the 

 associated igneous rocks which form separate and isolated masses, or are more or less entangled 

 in the stratified limestone. 



e. Specimens from the Organ Mountain range, fifty miles north of the locality of Nos. 40 and 41. 

 The elevation of this range is about 2,000 feet above the bed of the river. 



The specimens from the Organ Mountains consist of compact feldspathic granite with very 

 little quartz, a few scales of green mica and hornblende, and numerous minute crystals of mag- 

 netic oxide of iron. Notwithstanding the compactness of this mass, the character and mode 

 of aggregation are so similar to some of the well-characterized volcanic products that it can 

 scarcely be regarded as an ancient granite. Resting on this rock occurs very compact green- 

 stone porphyry. A specimen from the western base of the mountain is a reddish lava-like 

 porphyry with a finely porous or vesicular structure. 



42. Granitic rock, composed of crystalline feldspar with a smaller proportion of quartz and 

 hornblende. The specimen is less lava-like than No. 40, but it has the aspect of a very modern 

 igneous rock. This constitutes the central portion of the mountain range. 



43. Porphyritic greenstone, very compact, forming the mass partially surrounding No. 42. 

 This rock occurs in distinct layers dipping at an angle of 82° W. 



44. Brown porphyritic trap, overlying the granitic central mass of the mountain. 



45. Eeddish brown, compact, lava-like rock, containing minute crystals of feldspar ; associated 

 ■with Nos. 42, 43, 44, forming extensive masses. 



46. Sulphuret and phosphate of lead, sulphuret of copper, and sulphate of baryta. There are 

 several specimens all presenting the same general character, and obtained from a vein in the 

 mountain range. 



47. A coarse porphyry ; a red, coarse, loosely aggregated base. Near the San Antonio road. 

 48 Volcanic breccia. Near the San Antonio road. 



49. Amygdaloidal trap ; a common, grayish base with round vesicles. Tascate. 



50. Porphyry. 



