108 GEOLOGY AND PALAEONTOLOGY, 



26. Brown porphyry ; same as the preceding, except that it contains cavities, which are 

 probably due to the weathering out, or solution of crystals from the mass. 



2T. Chalcedony mixed with Feldspathic lava. This rock has been formed by gelatinous 

 silica penetrating scoria or other loose volcanic materials. 



28. Chalcedony. Two specimens, associated with the preceding rocks. 



The volcanic products present numerous modified conditions, from the effect produced by 

 silica' in solution, or in the gelatinous condition, having penetrated the mass. From this cause, 

 and from the effects of heated water, not only the lithological aspect, but the color of the rock 

 is often greatly modified. 



29. A siliceous stratified rock, apparently a highly altered sandstone. From the Rio Grande, 

 seventy miles below El Paso. 



30. A porphyritic trap-like rock, perhaps a sedimentary rock altered by volcanic action. 

 Seventy miles below El Paso. 



31. Greenstone trap. Seventy miles below El Paso. 



32. Volcanic breccia, a gray feldspathic mass with hornblende. Same locality as the last. 



33. Volcanic breccia, becoming porphyritic ; more compact and crystalline than the preceding. 



34. Granitic mass ; a volcanic granite, composed of feldspar and hornblende, with little 

 quartz ; probably a further modification of the breccia by igneous action. Seventy miles below 

 El Paso. 



The range east of the Kio Grande, seventy miles below El Paso, has furnished specimens of 

 reddish and greenish compact porphyry, compact trap, a specimen of granitic structure con- 

 sisting of feldspar, mica, and some earthy matter, but loosely aggregated, not unlike the 

 products of recent igneous formations ; also several specimens of volcanic breccia loosely aggre- 

 gated, and a single specimen of similar composition very compact in texture. From this 

 locality there is also a specimen of carboniferous limestone, partially crystalline, and one side 

 permeated by innumerable minute pores ; but it still preserves evidences of organic remains on 

 its weathered surfaces. 



d. Specimens from the Tertiary Basin of the Bio Grande. 



35. Selenite mixed with marl from a bluff composed of marl, gravel, and beds of selenite. 

 The crystals occur in detached groups, but altogether form large beds. 



36. Cretaceous limestone with Exogyra texana, and Serpula. El Paso. 



37. Argillaceous limestone with Inoceramus, from the Eio Grande above El Paso. 



38. Hornstone with seams, or a brecciated intermixture of limestone with hornstone, forming 

 the summit of a mountain 1,200 feet high. The base of the mountain is composed of the upper 

 carboniferous limestone. Near Frontera. 



39. Black hornstone, surrounded by a lighter colored mass of the same. Near Frontera. 



40. Feldspathic or granitic volcanic rock, composed mainly of Feldspar and Olivine. Two 

 specimens from near Frontera. 



41. Compact feldspar with a little glassy quartz : forming knobs and dykes running through 

 the limestone strata ; sometimes overlying and sometimes underlaying the latter rock. It is 

 evidently of volcanic origin. 



No. 40 forms isolated knobe less intiniatelv connected with the limestone than No. 41. 



