150 APPENDIX D. GEOLOGY. 



not in the United States." (p. 126.) Either deposite may be large 

 enough to supply the wants of the inhabitants who may live near enough 

 to obtain it. But the vast extent of your deposite (doubtless greater, 

 as you say, than is at present known) will make it accessible to much 

 the greatest number of people. Indeed, from the well known use of 

 this substance in agriculture, as well as other arts, a knowledge of its 

 existence must have an important bearing upon the settlement and pop- 

 ulation of northwestern Texas. 



The only deposites of gypsum known to me that are more extensive 

 than the one discovered by you, are in South America. All along the 

 western side of the Cordilleras, especially in Chili, and interstratified 

 with red sandstone and calcareous slate, beds of gypsum occur of enor- 

 mous thickness, some of them not less than six thousand feet. It has 

 been tilted up and metamorphosed greatly by igneous agency of ancient 

 elate, but seems to be of the age of tbe lower cretaceous rocks. Mr, 

 Darwin, to whose admirable work on the geology of South America I 

 am indebted for these facts, has traced this deposite at least five hundred 

 miles from north to south, (it is not many miles — sometimes, however, 

 twenty or thirty — in width,) and thinks it extends five hundred more; 

 and perhaps much further. He also describes thin beds of gypsum in 

 the tertiary strata of Patagonia and Chili, which are some eleven hun- 

 dred miles in extent. This gypsum is generally more or less crystalline, 

 and corresponds much better in lithological characters with that in 

 Texas, than does the metamorphic gypsum of the Cordilleras. Mr. Dar- 

 win is of opinion, however, that the latter was originally deposited in a 

 manner analogous to the former, viz : by means of submarine volcanoes 

 and the conjoint action of the ocean. Veiy probably the ancient igne- 

 ous agency which we have described in the Witchita mountains, and 

 along a line southerly to the Rio Grande, may have been connected 

 with the production of the gypseous deposite in the same region. 



The specimens of this gypsum put into my hands correspond with 

 your descriptions. One of them, of snowy whiteness and compact, it 

 seems to me, might answer for delicate gypseous alabaster, so extensively 

 wrought in other lands for ornamental purposes. The selenite was re- 

 garded among the ancients as the most delicate variety of alabaster, 

 and was employed by the wealthy, and in palaces, for windows, under 

 the name of Phengites. It has the curious property of enabling a per- 

 son within the house to see all that passes abroad, while those abroad 

 cannot see what is passing within. Hence Nero employed it in his 

 palace. If the splendid plates which you describe occur in any consid- 

 erable quantity, it may hereafter be of commercial value, as it certainly 

 will be of mineralogieal interest. 



