470 SCIENCE PROGRESS. 



from 51 to 61 per cent., and the alkalies jointly from SJ to 

 10 per cent, potash predominating. The most acid bana- 

 kites carry a certain amount of quartz. We have thus a 

 well-defined series of rocks which may be roughly described 

 as alkali-basalts and alkali-andesites. 



Passing southward we have next to notice another area 

 of rocks rich in alkalies. Just as the Geological Survey of 

 Arkansas gave us a valuable account of nepheline-bearing 

 and allied rocks in that State, so the Texas Survey has 

 discovered the existence of an extensive development of 

 such rocks in Southern and Western Texas. Kemp in 1890 

 recorded a Cretaceous nepheline-basalt from Pilot Knob, 

 near Austin, and since then a number of interesting- rocks 

 have been described by Osann. Three years ago he drew 

 attention to two types occurring in Uvalde County in the 

 southern part of the state (15). One, forming dykes, is a 

 fresh rock named melilite-nepheline-basalt, the two minerals 

 being present in about equal quantity. There are large 

 crystals of olivine, and the ground-mass consists of augite, 

 nepheline, melilite, magnetite, and perofskite. The other 

 type, forming hills and buttes, is a nepheline-basanite, in 

 which the porphyritic elements are hornblende, augite and 

 nepheline, with some felspar and olivine. Since part of 

 the felspar is a sanidine, and olivine is rare, the rocks 

 approach phonolite in characters. 



More recently the same geologist has described a varied 

 group of rocks, plutonic, intrusive and volcanic, in and 

 around the Apache Mountains in the western (trans-Pecos) 

 portion of Texas (16). An elaeolite-syenite occurs at 

 Paisana Pass, and another in the Mount Ord Range, to 

 the south-east of the Apaches. In both lavenite is a con- 

 stant accessory mineral. The Mount Ord rock passes from 

 a normal elaeolite-syenite through a fine-grained porphyritic 

 variety to a marginal phonolitic facies. The change of 

 texture and structure is accompanied by mineralogical 

 changes, the malacolite-like augite, hornblende, and mica 

 giving place to aegirine-augite, aegirine, arfvedsonite, and 

 aenigmatite. In the Saw-Tooth Mountains, in the western 

 part of the district, occurs an augite-hornblende-syenite, 



