110 GEOLOGY AND PALEONTOLOGY. 



51. A porphyritic rock with chalcedony. 



52. Compact, laminated porphyritic rock. 



53. Compact, micaceous, gneissoid sandstone, slightly calcareous; dipS.S.E. 60°. 

 This sandstone has the aspect of a cretaceous or tertiary sandstone. 



54. Laminated, compact, argillo-calcareous rock, which has been subject to igneous action, 

 and partially altered. 



The two preceding specimens are from strata overlaid by trappean rock, before noticed ; and 

 their altered condition is doubtless due to this action. This fact places the date of these erup- 

 tions subsequent to the cretaceous period, and perhaps posterior to the older Tertiary deposits. 



55. Encrinital, subcrystalline, light-gray limestone, containing numerous fossil fragments ; 

 belongs to the upper carboniferous period ; dip to the southeast. Cibolo creek. 



56. Argillaceous sandstone with mica, fine-grained and thinly laminated. The age of this 

 sandstone is doubtful, but it may be carboniferous. Cibolo spring. 



57. Compact porphyry. Cibolo creek. 



58. Limestone of the upper carboniferous period, forming high mountains ; dip southeast. 

 Presidio del Norte. 



This rock is similar to that from Cibolo creek, but more compact and less crystalline, and 

 evidently has undergone a partial metamorphism. 



A specimen from the rapids of the Del Norte is of a bluish ashen color, very compact and 

 fine-grained, with numerous crystalline points and lines which mark the presence of organic 

 bodies. Although no fossils can be recognized in their specific character, yet the rock is so 

 precisely of the character of the carboniferous limestone in numerous western localities as to 

 leave no doubt of its true age and geological position. 



The specimens from Buffa-silla, marked " Aug. 10," consist of the following : two specimens 

 of vesicular lava, one of compact breccia, enclosing fragments of lava, and several specimens 

 of trap tulF, enclosing quartz pebbles and fragments, and becoming, in one specimen, a sort of 

 friable breccia. Another specimen from the same locality, marked "Aug. 13," is a compact, 

 volcanic breccia, composed mainly of fragments of various volcanic materials. 



59. Vesicular lava, Buffa-silla. 

 A second similar specimen. 



60. Compact lava, Buffa-silla. 



61. Volcanic breccia, composed of fine white volcanic ashes, with pebbles and fragments. 



62. Coarser breccia, with a coarser base than the preceding. 



The compact lavas are represented as lying below, and the breccias between these and the 

 more cellular lavas above. Sometimes there are several successive series of these beds. 



63. Granitic or compact feldspathic lavas, with quartz and hornblende in small proportions. 

 Three specimens from the cafion of the Kio Grande. 



A specimen from the caiion of the Rio Grande, marked "Aug. 27," consists of a crystalline 

 aggregation of quartz, feldspar, and carbonate of lime. Two other specimens, same date, from 

 Puerto Peak, are granitic aggregations of quartz and feldspar, and appear much like a partially 

 fused breccia. 



64. Cemented volcanic ashes, with, occasionally, small vesicles, giving an amygdaloidal 

 character. 



