420 The American Naturalist. [May, 



beautiful crystals of aegirine, eleolite, eudialite, and other decomposi- 

 tion products of eleolite. This rock is the gangue of many of the 

 sphene, natrolite, brucite, manganopectolite and eucolite crystals 

 obtained in this region. Other dykes in which eleolite occurs in two 

 generations, sometimes with and sometimes without orthoclasc in the 



orthoclase are also described. But perhaps the most interesting of 

 the rocks of Magnet Cove are the leucite dyke rocks. Of these the 

 author distinguisues between leucite-syenite and leucite-tinguaite. The 

 former is a hypidiomorphic granular aggregate of a pseudomorph 

 after leucite, eleolite, orthoclase and the basic silicates, diopside and 

 biotite. It is generally connected with eleolite-syenite, and is easily 

 recognized by the large crystals of pseudo-leucite scattered through it. 

 These are imbedded in a ground mass in which may be detected eleo- 

 lite, black garnets and feldspar. No trace of leucite may be discov- 

 ered in the large crystals. They are now composed of tabular ortho- 

 clases, interspersed with small eleolites and pyroxenes. Within the 

 mass the orthoclase is radial, while • on its periphery smaU tabular 

 crystals have their symmetry planes perpendicular to its surfaces. The 

 ground mass has the structure of an eleolite porphyry. Tin tingufo 

 occurring as dykes possesses two generations of orthoclase, aegirine, 

 eleolite and the pseudomorphs of leucite, besides many accessory com- 

 ponents. It differs from the eleolite-tinguite mainly in the possession 

 of orthoclase in large quantity. Analyses of eleolite-tinguite, leuci e- 

 syenite, tinguite and of the pseudo-leucites follow: 



formed, but these are so indefinite in their character that but little 

 study. In the sandstones and 



\ : 



On the contact of the eruptives with the country rock 



formed, but these are so indefinite in their d 



could be learned from their microscopic study. 



limestones through which the eleolite-syenite ( 



important contact minerals, many of which are of world-wide int< 



Among those found in quartz rock and sandstone may be men I 



smoky and milky quartz, arkansite, rutile and hematite. r H ie 

 addition to the sodium given s 

 ,V>* of SO., and ASfc ° fil 



