MELILITE-NEPHELINE- BASALT AND NEPHELINE- 

 BASANITE FROM SOUTHERN TEXAS. 



These basaltic rocks were collected by Professor Dumble and 

 Mr. Taff, in Uvalde County, southern Texas. On the geological 

 map of the United States, compiled by C. H. Hitchcock, 1886, 

 there are two of the localities marked near the boundary of 

 the Cretaceous and earlier Tertiary formation, between 99° and 

 100° longitude, and on the 29th degree of latitude. According 

 to the statement of Professor Dumble, one part of the rocks 

 appears in dikes in the upper portion of the lower Cretaceo is 

 formation, while the other forms hills and buttes. Upon micro- 

 scopical examination it is evident that the specimens collected 

 belong to two different groups of rocks. The microscope shows 

 that those occurring in dikes consist of typical melilite-bear- 

 ing nepheline-basalt, while those making up hills and buttes 

 are nepheline-basanites tending toward phonolites in composi- 

 tion. 



The melilite-nepheline-basalts have a typical basaltic appear- 

 ance. In a dense black groundmass, the only phenocrysts seen 

 by the naked eye are numerous olivines. Under the microscope 

 there appear in addition to the olivine the following minerals: 

 augite, nepheline, melilite, magnetite and perovskite. As to the 

 proportion of nepheline and melilite, it can be said, that in 

 nearly all the specimens examined, the two^minerals are found in 

 about the same amount. For this reason these rocks can be 

 placed under the head of nepheline-basalt as well as under that 

 of melilite-basalt, or they may be called melilite-nepheline-basalt. 

 Only one of the specimens is entirely free from melilite. Feld- 

 spar is wholly wanting. All of the specimens are in a very 

 fresh condition, and even the melilite shows only slight indica- 

 tions of decomposition. The specimen [free from melilite corre- 



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