MEXICAN BOITNDARY LINE 111 



Three specimens, showing variety (of 64.) 



65. Breccia with a white base. 



66. Lava, less compact than that below. 



The series shows the succession of compact lava, breccia, and less compact, or vesicular lava 

 above. 



67. A fragment including part of a cavity in amygdaloid, with green quartz. 



68. Brownish porphyritic rock. This has the common character of the porphyry of the 

 region. 



69. Crystallized peroxide of iron, connected with the trap or lava deposits. 



70. Porphyritic granite, apparently of very modern origin. 



71. Sienite or porphyritic granite, varying but little from No. 70. It contains dark smoky 

 quartz. 



72. Reddish porphyry or porphyritic trap, associated with the preceding specimens. 



73. Compact argillaceous limestone of the Cretaceous formation. 



74. Compact, close, and fine-grained argillaceous limestone of the Cretaceous formation. 



75. Exogyra fexana, from above the Pecos river. 



76. Argillaceous limestone, with Exogyra, containing cavities filled with calcareous spar. 



77. Fossil wood from Tertiary strata ; Eagle Pass. 



II. Specimens from the country west of the Rio Grande. 



78. Sandstone, compact and fine grained. Cemialauke. 



This rock is said to form a mountain range in connexion with the conglomerates. The 

 specimens are not sufiicient to determine satisfactorily the geological age of the formation. 



79. Carboniferous limestone with fossil remains. 



These specimens, (79,) from west of Salado, have evidently undergone partial metamorphism, 

 though still preserving fragments of organic remains. One of them contains several imperfect 

 shells, among which a Terebratula is distinguishable. 



80. Porphyritic lava, connected with the preceding limestone ; and breccia, connected with 

 the same. 



81. Compact trap, with a silicious incrustation covering the surface. 



82. Amygdaloid. 



An extensive district is represented as covered by rocks like 81 and 82, on the southwest of 

 Frontera. 



8B. Amygdaloid, similar to the preceding ; one hundred miles west of El Paso. 



84. Specular iron ore. It occurs in loose masses, scattered over the Tertiary plains. 



85. Chalcedony, associated with trap rocks. 



86. Feldspathic lava, or compact trap tuff. 



Eocks of this character are represented as forming the dividing ridge and summit of the 

 Sierra Madre near the Gaudaloupe Pass. 



88. Quartz rock ; some portions are granular, showing the passage from an arenaceous mass 

 to a compact homogenous quartz rock. 



(This is, apparently a metamorphic stratified rock.) 



A specimen from the summit of the Gaudaloupe Pass presents the character of rounded 



